A histological investigation was made to elucidate whether mammalian (ovine) prolactin can induce mitotic activity in pancreatic islet cell (in splenic lobe) of chicks. Prolactin treatment in different dosages (total dosage: 15, 50 and 100 IU per bird in 10 days) increased the mitotic frequency and also caused hypertrophy in the AF-positive cells of the B-islets in the splenic lobe. The increase in mitotic frequency was uniform for all doses applied. It is suggested that prolactin has a mitogenic action on the pancreatic islet cells of male chicks and that action is possibly not dose-dependent.