Perhaps I just have had a lot of bad luck and bad timing, but I never saw this episode until today--and that's odd, as I thought I'd seen them all and watched the re-runs again and again over the years. I wonder if others have also noticed this.
Patrick O'Neal stars as a sad old rich guy who has married a beautiful young harpy. His "beloved wife" (Ruta Lee) deeply regrets marrying an old man and never hesitates to rub his age in his face. She's simply a horrible woman and yet he still loves her and wants to please her. In desperation, he takes an experimental medical formula to make him younger. Do things work out for him? Well, remember it IS "The Twilight Zone"! I found this show a bit predictable, as I quickly guessed what would happen once O'Neal took the formula. Still, it was well-acted and enjoyable, so this telegraphing can be forgiven. Worth seeing...and with a well-deserved twist.
By the way, although she plays a horrible person, I agree with Hitchcoc's review in regard to Ruta Lee in this show...Hubba, Hubba! And I've seen recent photos of her and wonder if she, too, took some of this miracle rejuvenation potion!