Diurnal variation of the symptomatology of endogenous depression (melancholy) was examined by means of 4 scales on 5 consecutive days in 24 untreated patients and in 63 patients under antidepressant drugs and after sleep deprivation. RESULTS the group means exhibit a sinus-type daily curve with morning low (but not significant). This so-called typical diurnal variation is more often found than other definable diurnal variations. The frequency cannot, however, be specified with one figure, because the quota varies, according to the criteria used, between 36.1 and 4.2%. The so-called typical diurnal variation is thus irregular, and intraindividually it also proved to be unstable. It is not specific for endogenous depressions. There is evidence of frequent shorter (ultradian) variations of depression symptomatology that are, however, difficult to record. Pharmacological therapy has little influence on the depressive diurnal variation. Sleep deprivation can revert the inverse diurnal variation and provoke a typical diurnal variation. After methodological deliberations the diagnostic and therapeutic implications are discussed.