The Flintstones - Season 5 Episode 3 -

Inventor Fred comes up with a weight-loss potion that doesn't just shed pounds, it drastically reduces Fred's size. Capitalizing on the weird situation, Barney goes on ""The Ed Sullystone Show"" as a ventriloquist, with Fred acting as his dummy. But during the act, Fred returns to his normal size, and they are thrown off the show.