Plasma PRL, cortisol, and GH responses to a standard iv dose of regular insulin were studied during the morning and evening in seven normal young men. Hypoglycemia achieved during morning and evening in the same aubjects was equal. There was a substantially greater maximal increment in PRL in the evening compared to the morning (P less than 0.01). The peak levels of cortisol achieved in the morning and evening were equal, but the evening maximal increase was greater (P less than 0.05) because of the significantly lower evening basal cortisol level. Evening increases in GH were greater than in the morning in five subjects and were essentially the same in two subjects; for the group, the evening maximal increment in GH was significantly greater (p less than 0.05 after log transformation). Since serotonergic mechanisms appear to be involved in the PRL, GH, and cortisol responses to hypoglycemia, we suggest the possibility of a diurnal variation in hypothalamic serotonin activity which may partly mediate these differential diurnal hormonal responses to hypoglycemia (although other neurotransmitters may also be involved). The data on cortisol are discussed with regard to the reset hypothesis of feedback inhibition.