During the last decade, circadian and episodic fluctuations of the plasma levels of pituitary hormones have been demon~ trated in man. The interest in the time dependence of the pituitary secretions partially originates from their close functional relation with the neuroendocrine mechanisms pre- sumably responsible for the control of the circadian.rhyth- micity in man. High frequency blood sampling techniques and specific radio- immunoassays have allowed to describe in detail the varia- tions over the 24-h cycle of the hormonal concentrations in healthy individuals under basal conditions as well as after manipulation of external synchronizers such as the sleep- wake cycle, the dark-light cycle and the feeding schedule. Alterations in normal hormonal var~ions have been sugges- ted to play a role in the etiopathogeny of several metabo- lic and mental diseases, such as obesity or manic-depressio~ Important clinical implications regarding treatment and pre- vention are expected to be developed in the near future. However, so far, data obtained in different groups are dif- ficult to compare because different hormones were studied using different protocols and statistical methods. As a result, unclear or controversial issues are not uncommon.