A 59-year-old depressed woman was studied in the sleep laboratory for 43 nights: 17 before leucotomy and the rest at various intervals up to six months after it. Preleucotomy sleep was characterized by scarce delta and REM sleep. Leucotomy did not immediately affect the patient's sleep stage distribution, but caused a temporary increase in wakefulness lasting two to three weeks. This result suggests the functioning of a forebrain inhibitory system in man similar to the one described by Sterman and Clemente for cats. A gradual trend toward more delta sleep began about ten days after leucotomy, and continued for about five months until age-appropriate levels of delta sleep were observed. Since the psychological benefits of leucotomy appeared immediately after the operation while the recovery of delta sleep was long delayed, it seems that the two variables are not directly related. Rather, it appears that the suppression of delta sleep in depressive illness is brought about by some third slow-to-change variable, possibly related to a gradual change in some neuroendocrine balance.