Famed producer Sherwood Schwartz decided to give Saturday mornings another try in 1976, this time with his first entry for NBC, Big John, Little John, a collaboration with the team of William D'Angelo, Harvey Bullock, & RA Allen (Monster Squad). 70skidvid provided the open & close to the show, uploaded to YouTube:
It was an odd concept. A middle-aged man (Herb Edelman, later of The Golden Girls) drinks from the fountain of youth, or so it would seem, leading to the unpredictable age-&-size-changing from adult to youth (with Robbie Rist, ex-The Brady Bunch, as the younger version) that formed the basis for this sitcom. However, airing opposite Krofft Supershow, and, I believe, Ark II, Big John couldn't find an audience. In hindsight, this whimsical comedy was meant for prime time consumption, but then, NBC couldn't buy a break at night, either, in those days.