The plasma concentrations of prolactin were measured by radioimmunoassay throughout the breeding cycle in female domestic ducks. Prolactin levels increased significantly during egg laying from 5 to 15 ng/ml and again at the start of incubation to about 28 ng/ml. Prolactin levels remained consistently high throughout incubation and fell sharply around the time the eggs hatched. Local anaesthesia of the incubation patches of incubating females for 9 hr caused a significant decrease in plasma prolactin. It is suggested that tactile stimulation from the eggs in the nest may cause the rise in prolactin during the egg laying period and also maintain prolactin secretion during incubation.