The role of GABAA and GABAB receptors in neuronal mechanisms of the baroreceptor reflex in the nucleus tractus solitarii (NTS) of the anesthetized and immobilized rabbits were investigated using a microiontophoretic technique. Baroreceptive neurons (BRNs) activated or depressed by baroreceptor stimulation (phenylephrine, 10 micrograms/kg, i.v.) were identified in the NTS (activated BRN (A-BRN) and depressed BRN (D-BRN), respectively). The GABAA antagonist bicuculline (40-80 nA) increased spontaneous activities of these neurons, but the GABAB antagonist phaclofen (80-160 nA) did not. Evoked responses of A-BRNs were potentiated by bicuculline and phaclofen, while the responses of D-BRNs were not clearly affected by these drugs. These results suggest that most of A- and D-BRNs are tonically inhibited by endogenous GABA acting on GABAA receptors, but not on GABAB receptors, and that GABAergic mechanisms suppressively modulate the baroreceptor reflex acting on GABAA and GABAB receptors of A-BRNs, but not of D-BRNs.