Season 2: Episode 11: Brian In Love
After cleaning up more than a few pee stains around the house, Lois thinks it may be time to potty-train Stewie. Stewie, outraged that he's being blamed for the carpet stains, totally resists Peter's efforts to train him. Meanwhile, Brian knows the truth - that he's the one responsible for the pee stains. He seeks professional help only to find that he can't hold his bladder because he's in love with Lois.