A differential reinforcement of low rate schedule was used with rats to test 15 psychotropic drugs. The computer analysis was based on interresponse time (IRT). Mean IRT, IRT standard deviation, median IRT, IRT midrange, modal IRT, frequency of modal IRT, and an efficiency index, in addition to numbers of responses and reinforcements and the IRT histogram were obtained for each rat in each drug test. An increase in number of responses and a peak shift to shorter IRTs in the histograms were observed with amphetamine, methamphetamine, priradrol and nicotine, as reported by many other investigators. Decrease in IRT midrange and less change in number of responses were observed with diazepam and chlordiazepoxide. Long pauses were found with LSD-25, 2,5-dimethoxy-4-methylamphetamine (DOM) and mescaline. In a factor analysis, the following main factors were obtained. High values in factor loading a1 were observed with chlorpromazine, chlordiazepoxide, pentobarbital, imipramine, nialamide, LSD-25, DOM and mescaline. With these drugs, mean IRT and IRT standard deviation were also high. Values for a2, were high with amphetamine, methamphetamine, pipradrol and nicotine. High a3 values were observed in some rats with chlorpromazine, diazepam, chlordiazepoxide, pentobarbital, pipradrol and caffeine. The changes in a3 values were correlated with changes in the IRT midrange. These results may be valuable in classifying new compounds in drug screening programs as being of the amphetamine type, nicotine type, diazepam type of LSD-25 type.