SummaryThe effects on lactation of perphenazine, a tranquillizer which increases the level of prolactin in the blood, have been studied in cows, goats, guinea-pigs and rabbits. The drug caused a significant depression of milk yield in cows, goats and rabbits, but was without effect on the milk yield of guinea-pigs. It seems probable that the inhibitory effects on lactation were due to the actions of the drug on appetite and behaviour. It is concluded that perphenazine, because of its other actions, is not a suitable agent for studying the physiological effects of increased levels of endogenous prolactin.