The iconic BBC children's television series ran from 1964 until 1988.
Johnny Ball, an ex-presenter of the show, said: "There was Rick Jones, Lionel Morton and myself. They got stoned on the biggest joint you've ever seen - in the studio. We were in silhouette as the three shepherds with our crooks. Lionel purposely held his crook so the crook didn't show.
"They were absolutely stoned out of their minds."
The 73-year-old, the father of Zoe Ball, added that he did not use drugs himself, saying: "I couldn't work with it. I really couldn't."
The Sun reports that drug use was also tolerated elsewhere in the BBC, with presenter Joan Bakewell saying of visiting pop acts: "Of course they smoked and they didn't smoke ordinary cigarettes."
BBC alumni also commented on sex at the BBC, with Katy Manning, who played Jo Grant on Doctor Who from 1971 until 1973, saying: "People were bonking all over the BBC. Everybody was doing it on the premises."
Janet Fielding - who played the Doctor's sidekick between 1981 and 1984 - added: "Nobody cared whether you had sex in your dressing room."
The BBC confessions will be screened in Tales of Television Centre on BBC Four on May 17.
Watch the intro to Play School from 1973 below: