The effects of pirlindole and dehydro-pirlindole on GABAA receptors and MAO-A activity were investigated in vitro. Pirlindole was inactive as a GABA antagonist. Dehydro-pirlindole exhibited partial and selective blockade of a subset of GABAA receptors with an EC50 of 12 microM and maximum reversal (delta Bopt) of 42%. Inhibition of rat brain and human placenta MAO-A by both compounds was much more potent (with IC50 range 0.3-0.005 microM). Their effects on MAO-A activity were partially reversible in vitro. In contrast to pirlindole, dehydro-pirlindole may act not only as MAO-A inhibitor but also as a clozapine-like selective GABAA receptor blocker, preferentially blocking a subset of GABAA receptors that are not sensitive to DMCM or Ro 5-4864.