The relative effectiveness of N,N-dimethyltryptamine derivatives and amphetamine derivatives was evaluated in seven squirrel monkeys, trained to discriminate between two black discs of different sizes. After the animals learned the task, they were injected intraperitoneally with the test compounds and then retested. The dose-response curves showed that debilitating effects of the compounds were a function of dose. The median effective doses (ED50) that disrupted their performance were calculated according to the graphic logarithmic probit method. The descending rank order of effectiveness of the compounds, according to the ED50 was: 2,5-dimethoxy-4-ethyl-amphetamine; 2,5-dimethoxy-4-methyl-amphetamine; psilocybin; 4-methoxy-N,N-dimethyltryptamine; N,N-dimethyltryptamine; and 6-hydroxy-N,N-dimethyltryptamine.