The effects of hallucinogens and their nonhallucinogenic analogs on the performance of rats on the Lashley-III underwater maze were evaluated. After the rats were trained to swim through the maze without an error, they were injected i.p. with test compounds or saline solution and tested at various intervals after the injection. A substantial number of the treated animals made one or more errors. The time of peak effect of lysergic acid diethylamide (LSD-25) and mescaline was 15 min after the injection; that of 2-brom-lysergic acid diethylamide (BOL-148) and of several trimethoxyamphetamines (TMA) was 30 min; and that of d-amphetamine was 45 min. The dose-response curves showed that the effect of the compounds was a function of dose. The median effective doses (ED 50) in μmol/kg were: LSD-25 (0.3); TMA-2 (12.8); BOL-148 (15.9); d-amphetamine (32.0); TMA (33.4); TMA-6 (46.0); TMA-3 (52.1); mescaline (91.0); and TMA-4 (94.0). The rank order of potency of the compounds is, in general, similar to that derived from the data on man