Glucose was found to exert an In vitro regulatory effect on prolactin secretion. Its role in the modulation of stimulated secretion of prolactin in man is, however, not clear. To evaluate the effect of hyperglycaemia on prolactin release, three stimulatory tests with different mechanisms of stimulation were employed. Healthy male subjects served as volunteers during submaximal exercise, TRH test (0.2 mg i.v.) and administration of haloperidol (2 mg i.v.). Glucose (100 g in 400 ml) or an equal volume of water was given 30 min before the tests. Blood for glucose and prolactin analysis was taken via an indwelling catheter. The plasma prolactin concentration increased in response to each of the stimuli applied. However, the prolactin increase during hyperglycaemia did not differ from values obtained in tests performed in normoglycaemia after water administration. These results indicate that prolactin release in healthy man is not modulated by hyperglycaemia.