1. Voltage-clamp recordings were made from neurones of vesical pelvic ganglia isolated from the rabbit urinary bladder. A rhythmic outward current, ISH, which corresponds to the spontaneous hyperpolarization, occurred at fairly constant intervals in fifty-eight of eighty-four neurones superfused with Krebs solution. The peak amplitude of the ISH was 0.5 +/- 0.2 nA (n = 48; mean +/- S.E.M.). 2. The ISH was eliminated in a Krebs solution containing nominally zero calcium and 12 mM-magnesium. Lowering the temperature of the superfusing solution from 36 to 22 degrees C also inhibited the occurrence of the ISH. 3. Bath application of caffeine increased the frequency of ISH. In contrast, ryanodine and procaine reversibly blocked ISH. 4. In thirty-four of fifty-eight neurones, the ISH was composed of two current components, an initial fast ISH with duration of 1-10 s and a slow ISH lasting 15-60 s. In the remaining twenty-four neurones, ISH showed only the fast component. 5. The fast ISH was associated with an increased membrane conductance and the slow ISH was associated with a decreased membrane conductance. The reversal potentials of the fast and the slow ISH were -88 +/- 7 mV (n = 4) and -30 +/- 6 mV (n = 4), respectively. 6. Tetraethylammonium (5 mM) and barium (1 mM) blocked the fast ISH but not the slow ISH. Intracellular caesium injected by ionophoresis through a Cs(+)-filled microelectrode blocked the fast ISH, without affecting the slow ISH. Apamin and (+)-tubocurarine selectively suppressed the fast component of the ISH. 7. Substitution of isethionate (67 mM) for chloride increased the amplitude of the slow ISH and shifted the reversal potential of the slow ISH to +1 +/- 8 mV (n = 5). A slow ISH with amplitude of 0.1-1 nA and was still observed in a low-sodium (26.2 mM) solution. The stilbene derivative, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid (SITS), a chloride channel blocker, suppressed the slow ISH. 8. These results suggest that ISH is composed of two distinct calcium-dependent currents, a fast ISH produced by activation of potassium conductance and a slow ISH produced by inactivation of chloride conductance. 9. The after-hyperpolarization (AHP) following the action potential was also composed of apamin-sensitive and insensitive spontaneous hyperpolarizing oscillations. The apamin-insensitive component of IAHP was increased by lowering external chloride activity, while it was depressed by SITS.