James Rado, who gave voice to the Age of Aquarius as the co-creator of “Hair,” the long-running hit that debuted in 1968 as Broadway’s first rock musical and featured a then-shocking scene of full nudity, died June 21 at a Manhattan hospital. He was 90.

The cause was cardiorespiratory arrest, said publicist Merle Frimark, a longtime associate and friend.

Rado was an actor and singer who saw dramatic possibilities in the emerging hippie culture of the 1960s. He and his creative collaborator, Gerome Ragni, developed the idea for “Hair” as a musical extravaganza that brought together several sentiments of the time, including opposition to the Vietnam War, sexual experimentation, the growth of self-expression and a celebration of the ideals of youth.

“I was a bit older, but I was very drawn to the idealism of the hippies,” Rado told Britain’s Telegraph newspaper in 2010. “I felt it was almost spiritual, a cause. People were communicating in their own way, they were letting their hair grow, trying to form a culture, a new way of living based on this notion of love, for humanity and for each other in person.”

The sketchy plot revolves around a journey of self-discovery by a group of hippies called the Tribe. One of their number, a sensitive young man grappling with his place in the world, is drafted and sent to war. Rado and Ragni wrote the “book,” or the play’s dialogue, and the song lyrics. The music was composed by Canadian-American musician Galt MacDermot.

“Hair” opened at producer Joseph Papp’s off-Broadway Public Theater in 1967. (Papp would not allow an originally planned nude scene.) After considerable revision, the musical moved to Broadway in 1968, with Rado and Ragni acting in the two principal roles as Claude and Berger, respectively.

The play, which was subtitled “The American Tribal Love Rock Musical,” contained explicit four-letter words, same-sex kissing, a multiracial cast and a soon-to-be infamous scene at the end of Act I, in which members of the cast shed their clothes and stood facing the audience for a few seconds in subdued lighting.

The playwrights learned that there was no law in New York against nudity on stage as long as the performers were standing still. Actors were not required to appear in the scene but received a $10 bonus for doing so.

The public reaction to “Hair” ranged from laudatory to puzzled to apoplectic, but it was unquestionably a sensation. Newsweek critic Jack Kroll said the show “ignites the key images and issues of the lost-and-found generation . . . into a vivid uproar that has more wit, feeling and musicality than anything since ‘West Side Story.'”

“Hair” ran on Broadway for more than four years, then had an even longer run in London. Rado, Ragni and MacDermot won a Grammy Award for the cast album, which topped the charts in 1969 and sold more than 3 million copies.

The musical begins with actors walking through the audience, converging on stage as they break into the upbeat opening anthem, “Aquarius”:

Four songs from “Hair” reached Billboard’s Top 5, including “Aquarius/Let the Sunshine In,” which spent six weeks at No. 1 and won a Grammy Award as record of the year for the Fifth Dimension. Other hits included “Hair” by the Cowsills, “Good Morning Starshine” by the one-named singer Oliver and “Easy to Be Hard” by Three Dog Night.

“Hair” embodied the zeitgeist like nothing else of its time. It appeared before other high-concept rock musicals such as “Tommy,” “Jesus Christ Superstar,” “Godspell” and “Rent” and became a theatrical touchstone of its era, much as “Hamilton” would 50 years later.

In addition to bringing rock music to Broadway, “Hair” introduced several technical advances in lighting, smoke effects and other forms of stagecraft. The original Broadway cast included future stars Diane Keaton and Melba Moore. The show toured the world, and resident companies were formed in a dozen cities for long-running productions.

Newsweek’s Kroll called it “the greatest global cultural event of the ’60s.”

James Alexander Radomski was born Jan. 23, 1932 — he was an Aquarian — in the Los Angeles neighborhood of Venice Beach and grew up in Rochester, N.Y., and Washington. His father, a sociologist and onetime college professor, was a federal official. His mother was a homemaker who encouraged her son’s interest in theater.

Rado, who later shortened his last name, studied at Catholic University and the University of Maryland, from which he graduated in 1954. He starred in a U-Md. production of “Romeo and Juliet,” acted in other campus productions and helped write several plays as a student.

He said he harbored ambitions from childhood of writing musicals in the vein of Cole Porter or Richard Rodgers and Oscar Hammerstein II.

After two years in the Navy, Rado moved to New York in 1956 to pursue a theatrical career. He studied with acting coach Lee Strasberg and, in the early 1960s, formed a singing group called James and the Argyles. (He and his male backup singers wore kilts and argyle knee socks.)

His early acting credits included a part in “Marathon ’33,” written and directed by June Havoc and starring Julie Harris. In 1966, while writing “Hair,” Rado was in the original Broadway production of “The Lion in Winter,” in the role of Richard, the son of the royal couple, played by Robert Preston and Rosemary Harris.

As a writer, Rado could never equal the success he found with “Hair.” He worked on a musical with his brother, Ted Rado, in the 1970s and later collaborated with Ragni on another project that did not made it to Broadway. In 2009, all three composers of “Hair” were named to the Songwriters Hall of Fame. (Ragni died in 1991; MacDermot died in 2018.)

Rado’s survivors include his brother.

In later productions of “Hair,” Rado returned more or less to the original Broadway version of the script. Something about the show seemed to resonate through the years, especially with young people facing social unrest and war.

“I think the reason it still works today,” Rado said in 1993, “is that it managed to capture a rare point in time when the philosophy of personal freedom was put into practice. That freedom provided the right to experiment and there were very fine things to remember about the era, to be learned. ‘Hair’ captures the best and still communicates it today.”