Exposure of cultured rat cortical astrocytes to increased concentrations of ammonia has been shown to induce morphological and biochemical changes similar to those found in hyperammonemic (e.g., hepatic) encephalopathy in vivo. Alterations of electrophysiological properties are not well investigated. In this study, we examined the effect of ammonia on the astrocyte membrane potential by means of perforated patch recordings. Exposure to millimolar concentrations of NH4Cl induced a slow dose-dependent and reversible depolarization. At steady state, i.e., after several tens of minutes, the cells were significantly depolarized from a resting membrane potential of -96.2 +/- 0.6 mV (n = 83, S.E.M.) to -89.1 +/- 1.6 mV (n = 7, S.E.M.) at 5 mM NH4Cl, -66.3 +/- 3.6 mV (n = 9, S.E.M.) at 10 mM NH4Cl and -50.4 +/- 2.5 mV (n = 12, S.E.M.) at 20 mM NH4Cl, respectively. In order to examine the underlying depolarizing mechanisms we determined changes in the fractional ion conductances for potassium, chloride and sodium induced by 20 mM NH4Cl. No significant changes were found in the fractional sodium or chloride conductances, but the dominating fractional potassium conductance decreased slightly from a calculated 0.86 +/- 0.04 to 0.77 +/- 0.04 (n = 9, S.E.M.). Correspondingly, we found a significant fractional ammonium ion (NH4+) conductance of 0.23 +/- 0.02 (n = 10, S.E.M.) which was blocked by the potassium channel blocker barium and, hence, most likely mediated by barium-sensitive potassium channels. Our data suggest that the sustained depolarization induced by NH4Cl depended on changes in intracellular ion concentrations rather than changes in ion conductances. Driven by the high membrane potential NH4+ accumulated intracellularly via a barium-sensitive potassium conductance. The concomitant decrease in the intracellular potassium concentration was primarily responsible for the observed slow depolarization.