The effects of the cholinergic blocker scopolamine on performance of rats in an 8-arm radial maze were studied. In Experiment 1, rats received injections of scopolamine-HBr (0.2 mg/kg, IP) or saline, 20 min before a trial. The drug impaired performance only when midtrial delays were introduced (10 sec, 5, 20, 60 or 120 min), during which the animals were removed from the apparatus, and to a similar extent at all delays. Injection of scopolamine directly after choice 4 in a 20 min delayed trial affected performance at a dose of 0.4 mg/kg, but not at 0.2 mg/kg. In Experiment 2, the rats were kept in the apparatus during the delay. There was a significant effect of scopolamine at 0.1 and at 0.2 mg/kg, initially irrespective of the length of the delay (10 sec, 2.5 or 5 min). After considerable training, administration of scopolamine (0.2 mg/kg) had no significant effect at the 10 sec delay but did impair performance after longer (5 or 10 min) midtrial intervals. These results suggest that scopolamine has differential effects, depending on dose and degree of training, and that an effect on memory storage may be one of them.