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.