As reported in the preceding study, the ability of certain antipsychotic and adrenolytic agents to inhibit the spontaneous firing of serotonergic 5HT neurons in the dorsal raphe nucleus appeared to be related to adrenergic blocking efficacy. However, the interaction between adrenergic and serotonergic systems was apparently indirect. In this phase of the study we investigated the hypothesis that another transmitter system could mediate this interaction. We examined the effects of two inhibitory amino acid transmitters (GABA and glycine) for possible effects on dorsal raphe cell firing using single cell recording and microiontophoretic techniques. In addition, the ability of the GABA antagonist, picrotoxin and the glycine antagonist, strychnine to reverse the effects of the antipsychotic and alpha-blocking drugs on dorsal raphe firing was tested. Both GABA and glycine were found to inhibit raphe cell firing selectively, allowing for a possible neurotransmitter function for these amino acids within the dorsal raphe nucleus. However, picrotoxin but not strychnine was found to reverse the effects of the antipsychotic and alpha-blocking drugs on raphe firing. Based on these results, we propose that the adrenergic input may influence 5HT neurons indirectly via a GABAergic interneuron or interposed GABA neuron.