George Chakiris was originally hired by Roger Corman to play "Black Jack" (later changed to "Heavenly Blues" by Peter Fonda), but insisted that a stunt double do his motorcycle riding, so Corman replaced him with Fonda, who was originally cast as "Loser".
Actress Laura Dern (daughter of Bruce Dern and Diane Ladd) was conceived during the movie shoot. Growing up, she sometimes showed people who asked about her parents a still of them in their biker outfits.
The trivia item below may give away important plot points.
Around the time of shooting the funeral for 'Loser' outside of the church at North Argyle, Bruce Dern was assaulted and beaten by two real Hell's Angels; Dern tried to explain he was in a movie, but the bikers didn't believe him and knocked him out cold on the sidewalk. Later, Bruce said: "It was, like, hey, I'm not even alive in the movie, I'm dead, and they're STILL beating on me..."