The Impatient Patient (1942)

Telegram deliverer Daffy Duck is in a swamp, with a message for someone named Chloe, when he starts hiccuping. Unable to stop his hiccups, Daffy decides to seek medical help, in a old house belonging to a Dr. Jerkyl, who advertises his services on a neon sign atop his house. Examining the hiccuping Daffy, Jerkyl decides to use a "scare cure" and chemically transforms himself into a goofy lug named Chloe, to whom Daffy had been assigned to deliver the message - a birthday greeting. Chloe chases Daffy around the house, until Daffy uses a syringe to squirt formula into Chloe's mouth, transforming him into a mischievous infant.