Pyrazidol (PRZ), 10 mg/kg, stimulated the flexor reflex of hind paw of the spinal rat in approx. 40% of preparations. This stimulation was abolished by a central serotonin blocking agent, cyproheptadine, but unchanged by an agent inhibiting the uptake of 5-hydroxytryptamine, clomipramine. At higher doses, which did not stimulate the flexor reflex, PRZ prevented the stimulation of flexor reflex produced by fenfluramine, but did not affect the stimulation produced by quipazine or clonidine. PRZ did not affect either the body temperature in rats at high ambient temperature or the fenfluramine induced hyperthermia. Therefore, PRZ shows characteristics of an agent potentiating serotonergic transmission most probably as the consequence of the inhibition of 5-hydroxytryptamine uptake.