Pirenzepine, (5,11-dihydro-11-[(4-methylpiperazin-1-yl)-acetyl]-6H-pyrido-[2,3] [1,4]-benzodiazepin-6-one dihydrochloride), tested on rats and mice, did not demonstrate any conspicuous behavioral action: it did not counteract reserpine hypothermia in mice, the L-DOPA hypermotility of mice, and (with the exception of very large doses) the amphetamine hypermotility in mice and rats. The drug neither prolonged the time of immobility of rats in the behavioral despair test, nor affected the central serotonin system in rats in tests for 5-hydroxytryptophan-induced head twitches, tryptamine-induced convulsions and fenfluramine-induced hyperthermia at high ambient temperature. Pirenzepine did not affect either the hind limb flexor reflex in the spinal rat, nor the action of serotoninomimetics of it. The investigated compound had strong peripheral cholinolytic action as it inhibited salivation and lacrimation in the oxotremorine test. The oxotremorine tremor was weakened only by very high doses of pirenzepine. LD50 of the drug in mice was 412 mg/kg ip.