In order first to establish the disappearance t 1/2 of endogenous immunoreactive human beta-MSH (RIA-h beta MSH), which is now known to represent the lipotropins, human beta-LPH and human gamma-LPH, the plasma concentrations of RIA-h beta MSH, RIA-hACTH, and fluorogenic corticosteroids (cortisol) were measured before and during infusion of cortisol in three patients with primary adrenocortical insufficiency from whom steroid replacement had been withdrawn. The t 1/2 for RIA-h beta MSH ranged from 44-133 min (mean, 83), and for RIA-hACTH ranged from 20-51 min (mean, 40). On this basis, 30-min sampling intervals were chosen to measure plasma concentrations of RIA-h beta MSH, RIA-hACTH, and cortisol in three normal subjects for 24 h. Similar diurnal rhythms were observed for all three hormones. Highest values (58.0 +/- 5.8 pg/ml) for RIA-h beta MSH were observed at about the time of awaking and lowest values (15.0 +/- 2.1 pg/ml) shortly after retiring. The secretion of both RIA-h beta MSH and RIA-hACTH appeared to be episodic, with about 8-10 secretory episodes/24 h, and the timing of their secretory episodes appeared to be nearly identical. There was excellent correlation (P less than 0.001) between simultaneous concentrations of RIA-h beta MSH and RIA-hACTH in all three subjects. These results establish the presence of a diurnal rhythm in plasma lipotropins (RIA-h beta MSH) and suggest that roughly equimolar amounts of ACTH and LPH are secreted simultaneously by the pituitary under basal conditions in man.