Seven volunteers received diazepam for a 3-week period, maximum dose rate 25 mg per day, and were tested on a variety of functions before, at two points during, and at two points after the period. Their performance was compared with that of 8 controls and was inferior on a number of measures. There was no indication either of habituation or of potentiation of the effects as the period continued. Equally, the low performance persisted for some time after administration ceased, although performance was on the whole better than during the experimental period. The particular measures showing deterioration suggested that the effects are similar to those of barbiturates as opposed to those of, for example, chlorpromazine.