The open field test used to study the behavioral alterations induced by psychotropic drugs is based mainly in the defecation, ambulation, rearing and grooming scores presented by animals subjected to the test. Because of the criticisms raised against the defecation score as a measure of the central effects of drugs, in the present experiment a modification of the test is proposed for the rat. The main points that characterize the new procedure are: (1) defecation scores are not considered; (2) besides ambulation, rearing and grooming, immobility was also recorded; (3) the total time of observation was increased, and (4) the stimuli, usually presented simultaneously in the open field (light and noise), were presented separately. The results obtained suggest that it is possible to differentiate classes of psychotropic drugs, without taking the defecation and grooming scores into consideration. Besides stimulants that evoked a characteristic pattern of behavior, neuroleptics could be differentiated from anxiolytics. Similarities, according to the dose, between barbiturates and anxiolytics were detected. Under LSD a peculiar pattern of behavior characterized by a large reactivity to light was observed.