Effects of ID-540, a new benzodiazepine derivative, on operant behavior were studied and compared with those of diazepam in rats for the purpose of determining the characteristics on behavioral pharmacology. Four schedules used were as follows: Fixed interval (FI-60sec) of food reinforcement and differential food reinforcement of low rate (DRL20sec) for positively reinforced behavior, Sidman-type avoidance response for negatively reinforced behavior and conflict behavior induced by simultaneously rewarding with food and punishing with electric shock. In the experiments on FI-60sec schedule, the responses at the early stage (0 approximately 30 min after administration of the drug) were increased by both ID-540 and diazepam at lower doses (0.5 approximately 4 mg/kg p.o.), but inhibited at higher doses (8 approximately 32 mg/kg p.o.). The effect of ID-540 lasted longer than that of diazepam. In the experiments on DRL20sec schedule, neither drug accelerated the responses, but decreased the lever-press response and total number of reinforcements at higher doses (4 mg/kg or more) showing the disturbance of discrimination on time. In Sidman-type avoidance responses, ID-540 did not show any inhibitory effect, thus a neuroleptic-like effect of ID-540 was not demonstrated. In experiments on FI-60sec and Sidman-type avoidance schedules, the effect of ID-540 was not changed by a consecutive administration for 10 days. Conflict behavior is considered to resemble the anxiety states in humans, and in related experiments, ID-540 increased the lever-press response which delivered a food-pellet and an electric shock simultaneously at a dose of 0.0625 mg/kg (i.p.). Change in other behavior was not observed at this dose level. Maximum effect of ID-540 was observed at a dose of 0.5 mg/kg (i.p.). Maximum effect of diazepam on conflict behavior was seen at a dose of 4 mg/kg (i.p.). The potency of ID-540 on conflict behavior was estimated to be about 8 times that of diazepam.