Abstract The effects of imipramine, chlorimipramine, desipramine and protriptyline have been studied on some central 5-hydroxytryptamine (5-HT) mechanisms. Central 5-HT receptor activity was estimated in spinal rats by studying the extensor hindlimb reflex. In the same rats, intra- and extraneuronal amine levels were semiquantitatively evaluated with the histochemical fluorescence method. It was found that imipramine and particularly chlorimipramine potentiated the effects of 5-hydroxytryptophan, tryptophan and nialamide on the extensor hindlimb reflex. If the animals had been pretreated with reserpine and nialamide, chlorimipramine caused potentiation of the effects of 5-hydroxytryptophan in doses as low as 1 mg/kg. Chlorimipramine did not alter this reflex in animals depleted of 5-HT. The increase in extensor hindlimb reflex activity was correlated with decreased amine levels in 5-HT neurons and sometimes associated with the appearance of 5-HT in the extraneuronal space immediatlely surrounding 5-HT cell bodies and terminals. Desipramine and protriptyline, in the doses used, lacked marked actions on this central 5-HT receptor.