Plasma prolactin concentrations in ad libitum fed male rats peaked at 19.00 h. However, prolactin concentrations peaked at 05.00 h and again at 19.00 h after two weeks of being fed between 08.00 and 10.00 h. Both peaks persisted even after six weeks of feeding between 08.00 and 10.00 h, suggesting that prolactin secretion is affected both by the feeding regimen and by the light-dark cycle. It is also suggested that prolactin may play an important role in the feeding behavior in the rat as it does in the bird.