1. Voltage-activated H(+)-selective currents were studied in cell-attached and excised, inside-out patches of membrane from rat alveolar epithelial cells in primary culture. The pH was varied from 5.5 to 7.5 in the pipette (pHo) and in the bath (pHi). 2. H+ currents in cell-attached patches exhibited activation kinetics and voltage dependence intermediate between their behaviour after excision into pH 6.5 and pH 7.5 solutions, consistent with reported pHi values for alveolar epithelial cells. 3. In inside-out patches, increasing pHo shifted the threshold voltage for activating H+ currents and the relationship between the time constant of activation of H+ currents (tau act) and voltage (V) negatively along the voltage axis by 40-50 mV (unit pH)-1. 4. Decreasing pHi shifted the activation threshold and the tau act-V relationship negatively along the voltage axis by 40-50 mV (unit pH)-1. In addition, at lower pHi the activation of H+ currents upon patch depolarization was markedly faster, with tau act increasing approximately 4- to 5-fold per unit pHi. 5. The limiting slope H+ conductance increased only by a factor of 1.7 per unit pH when pHi was lowered in the range 7.5-5.5 (at constant pHo 7.5). This result suggests that H+ permeation through voltage-activated H+ 'channels' occurs by a mechanism distinct from H+ permeation through water-filled ion channels, in which the conductance might be expected to increase in direct proportion to [H+].