The spontaneous firing rate of the noradrenergic neurons of the locus coeruleus and of the serotonergic neurons of the dorsal raphe was recorded with extracellular microelectrodes in chloral hydrate-anesthetized rats. A quantitative comparison of the effect of five tricyclic antidepressants, of tranylcypromine and of mianserin on the spontaneous activity of these two types of cells was performed. All drugs tested, except mianserin reduced the frequency of discharge of the noradrenergic neurons. Intravenous perfusion of the drugs allowed the doses required for inhibition of firing to 50% of the baseline rate (ID50) to be determined. Secondary aminated antidepressants (desipramine and nortriptyline) were more potent inhibitors than their tertiary aminated analogues (imipramine, chlorimipramine and amitriptyline). All drugs tested, except desipramine decreased the rate of firing of the serotonergic cells. In this case, the tertiary aminated antidepressants were much more potent than their secondary analogues. Mianserin was only active at very high doses. These results are in good agreement with the relative potencies of the tricyclic antidepressants for blocking the uptake of noradrenaline and serotonin into central and peripheral neurons.