Ten endogenous depressive patients were deprived of sleep for two whole nights according to the following schedule: sleep/sleep deprivation/sleep/sleep deprivation/sleep. No drugs were administered. Experimental conditions were as neutral as possible. Blind and nonblind ratings were taken. The patients were generally rated as improved after sleep deprivation, but a substantial effect, though temporary with rapid relapse, occurred in only two cases. After subsequent sleep, relapse followed as a rule. The net antidepressive effect of the total procedure was slightly more than nil.