Neurons in the rat rostral ventrolateral medulla (RVLM) were electrophysiologically characterized and identified using an intracellular recording technique in vitro. The recorded neurons could be classified into three types: spontaneously active neurons with a regular pattern of action potential generation; spontaneously active neurons with an irregular pattern of discharge; and silent neurons. In regularly firing neurons during hyperpolarization below spike generation level there occurred: (a) a 'resetting' of regular pattern of firing; (b) the absence of underlying excitatory postsynaptic potentials; (c) an anomalous rectification that produced a decay in the hyperpolarization. In regularly firing neurons, gamma-aminobutyric acid (GABA) (2-5 microM) produced a reversible membrane hyperpolarization, reduction of frequency of discharge and a moderate decrease in membrane input resistance. These effects were completely blocked in the presence of the GABAa antagonists bicuculline (16 microM) or picrotoxin (50 microM). However, the superfusion of bicuculline (16 microM), or picrotoxin (50 microM) alone elicited depolarization, increase in firing rate and increase of membrane input resistance. This study has provided evidence for regularly firing neurons in the RVLM in vitro, with strikingly similar electrophysiological characteristics to a group of neurons described in vivo as tonic sympathoexcitatory. In vitro they are still modulated by gabaergic inputs acting predominantly upon GABAa receptors.