The 24‐hour profile of plasma levels of immunoreactive β‐MSH (IR‐βMSH), ACTH and cortisol was obtained at 15‐min intervals in six normal males in summer and winter. In the radioimmunoassay used, dilution curves of human β‐MSH and human β‐LPH were not parallel. A seasonal variation in basal pituitary‐adrenal secretion, with higher levels in winter than in summer, was demonstrated. A circadian rhythm was found to be significant for ACTH, IR‐βMSH and cortisol in all investigations. Whereas ACTH and cortisol patterns were largely concordant in all studies, there was a significant desynchronization of the circadian rhythm of IR‐βMSH as compared to ACTH in five cases. Eighty‐ three percent of the secretory spikes of cortisol but only 68% of the IR‐βMSH spikes were concomitant with an ACTH spike. Correlations between maximal levels of concomitant spikes were higher during the quiescent period of pituitary‐adrenal secretion (22.00–04.00) for ACTH‐cortisol whereas for ACTH‐ IR‐βMSH, highest correlations were found during the active early morning secretory phase (04.00–10.00). For the three plasma constituents studied, longer apparent half‐lives were found to occur when the basal level before spiking was already elevated, suggesting that ACTH, β‐LPH and cortisol are secreted in bursts superimposed on a continuous basal secretion. Absolute increments of concentration appeared to be relatively independent of the level before spiking. It is suggested that the dissociations between ACTH and IR‐βMSH fluctuations in plasma observ