30 rock family portrait '30 Rock': Alan Alda marvels at the stamina of fellow guest Elaine Stritch

Jack Donaghy’s (Alec Baldwin) world spun off its axis when he discovered his biological father was a physically affectionate liberal named Milton Greene — and not the cold, demanding Bostonian he grew up with.

Naturally, after organizing a telethon to give Milton a kidney in “30 Rock’s” Season 3 finale, Jack more or less shunned him. This is particularly unfortunate given he’s played by “M*A*S*H” star Alan Alda.

Alda’s season-and-a-half absence comes to an end during Dec. 9’s ominously titled “Christmas Attack Zone,” and this time he gets to play opposite Jack’s mother, Colleen (Elaine Stritch).

“She’s remarkable,” Alda tells press of his 84-year-old on-screen baby mama. “She was coming in and shooting for a full day, and then she was going out at night to do a Broadway musical, ‘A Little Night Music.'”

“To do a musical takes a tremendous amount of energy,” Alda say. “She was doing two full time jobs at the same time.”

At least for most of her “30 Rock” work she gets to sit down. A scripted health scare meant she got to film some of her scenes in a hospital bed.

As for Alda’s return to the show, the actor says he sees a few similarities between “30 Rock” and “M*A*S*H.” “It’s similar to us in that they’re taking chances, and we took chances,” says Alda. “On this program, I think they’re expanding to another kind of comedy that’s risky. The people who love it, I think recognize how interesting and funny, in a new way, it is.”