Effects of penfluridol, a diphenylbutylpiperidine type psychotropic agent, on operant behavior were investigated and compared with those of chlorpromazine and haloperidol in rats trained on the following 5 schedules. Fixed ratio (FR 30) of food reinforcement and differential water reinforcement of low rate (DRL 20 sec) schedules were used for positively reinforced behaviors. Continuous (Sidman-type) and discriminated avoidance schedules were used for negatively reinforced behaviors. Conditioned suppression of response (CER) under FR 30 developed by stimulus presentation with electric shock was also applied. When 1 mg/kg of penfluridol was given orally, no change was observed in respondings of all the performances. At higher doses (2 approximately 8 mg/kg, p.o.), the respondings were inhibited in proportion to the dosage except in DRL performance, in which only correct response rate decreased at 8 mg/kg. These inhibitory effects were observed more apparently in the negatively reinforced behaviors than in the positively reinforced ones. Furthermore, a clear dose-effect relationship was obtained in the former. CER was not all attenuated by 2 approximately 8 mg/kg of penfluridol, thus a diazepam-like effect was not confirmed. These behavioral effects suggested that penfluridol has neuroleptic properties similar to those observed with chlorpromazine or haloperidol. However, in general, the inhibitory effects reached the maximum level approximately at 16 hr and lasted for 2 approximately 3 days after oral administration of the drug. Intensity of the effect of penfluridol was estimated to be about 1/8 approximately 1/10 that of haloperidol, according to the results obtained in the avoidance performances.