Continuous examinations of circulatory functions in three 24-hour periods,before, during and after sleep deprivation, were carried out on 26 endogenous depressives and 10 neurotic depressives. The blood pressure recordings scarcely revealed relevant findings. The diurnal rhythm of heart rate in endogenous depressives is less pronounced than in healthy subjects, a phasal shift not being noted. A correlation between diurnal variation of depression symptomatology and the circadian rhythm of heart rate could be presumed on the basis of these findings. The heart rate curve is further flattened, almost levelling off, on sleep deprivation. The extent of this change is related to the therapeutic effect of sleep deprivation. A less pronounced diurnal rhythm of heart rate was also noted in neurotic depressives; however,there was no relationship between the therapeutic effect of sleep deprivation and its influence on heart rate.