Male Sprague-Dawley rats were trained to press a lever on a simple-alternation multiple fixed-ratio (FR) 20-response time-out (TO) schedule for water reinforcement. Twelve 5-min periods of FR reinforcement were each followed by a 5-min TO in which responding had no scheduled consequence. Doses ranging from 0.25 to 8 mg/kg of eseroline, the hydrolysis product of eserine and a potent analgesic agent with weak anticholinesterase activity, were administered SC immediately prior to a 120-min test session. Eseroline produced a dose-dependent monotonic decrease in the number of reinforcements, with significant effects at doses of 1, 2, 4 and 8 mg/kg and an ED50 of 2.5 (1.6-3.4) mg/kg. This behavioral disruption was characterized by a rapid onset of pausing (i.e., within 5 min postdosing) and a gradual recovery to normal baseline levels of responding over the remaining session time. The duration of the rate decreasing effects was dose-related with the highest dose having a mean duration of more than 60 min, which was longer than that of previous reports on antinociception produced by eseroline (less than 60 min). The coadministration of behaviorally inactive doses of the opiate antagonist naloxone (1 and 2 mg/kg, IP) with eseroline (2.5 mg/kg, ED50) antagonized the effects of eseroline on the operant behavior. The coadministration of behaviorally inactive doses of the muscarinic antagonist atropine with eseroline (2.5 mg/kg) did not affect eseroline's behavioral effect. These results suggest that the effects of eseroline on operant behavior is consistent with the effects of eseroline-induced antinociception, reported previously, and appears to be associated with the activation of opiate receptors, but not related to the stimulation of muscarinic receptors via its anticholinesterase activity.