1. Whole-cell patch recordings were obtained from supraoptic nucleus (SON) neurones in horizontal brain slices of adult male rats. Low-Ca2+ or Ca(2+)-free perifusion medium induced oscillatory bursting activity in all sixty-nine cells displaying both phasic firing and depolarizing after-potentials (DAPs). In fifteen non-phasic cells without DAPs, Ca(2+)-free medium produced little or no oscillatory bursting. 2. Typical bursts started with rapid membrane depolarization, resulting in a plateau with superimposed action potentials, and ended several hundred milliseconds later in swift repolarization. Prominent bursting was observed at membrane potentials from -50 to -70 mV, with maximum amplitudes of 12.2 +/- 0.7 mV (mean +/- S.E.M.) around -70 mV. Development of oscillatory bursting was dependent on reduction of [Ca2+]o, with a threshold for the bursting < or = 1.2 mM Ca2+. 3. Bursting was abolished by addition of TTX, Co2+, Ni2+ or Mg2+ into the Ca(2+)-free medium, or by replacement of external Na+ with choline or Li+. Low concentrations of TEA or increased [K+]o prolonged burst durations and enlarged oscillation amplitudes. 4. Voltage-clamp techniques were used to examine the persistent Na+ current (INaP), and revealed that low [Ca2+]o shifted the threshold for INaP activation in a negative direction and enhanced the amplitude of this current. These changes in INaP were abolished by adding Co2+ or Mg2+ to Ca(2+)-free medium. 5. Direct diffusion of BAPTA or heparin into neurones or bath application of ryanodine suppressed bursting. Oscillations were also eliminated by the uncoupling agents heptanol, halothane or acidification. 6. CNQX, APV, bicuculline, CGP35348 (GABAB receptor antagonist), promethazine, atropine, d-tubocurarine and suramin had no obvious effects on oscillatory bursting. Blockers of transient Ca2+, or hyperpolarization-activating cation currents also did not alter bursting activity. 7. These results suggest that intrinsic burst activity in SON neurons perifused with low-Ca2+ or Ca(2+)-free medium involves enhanced Na+ influx through persistent Na+ channels, and requires the presence of rapid intracellular Ca2+ mobilization that might also explain the selective existence of oscillatory bursting in phasically firing cells. Intercellular communication through gap junctions appears to be important in determining neuronal activity of the neuroendocrine cells in low-Ca2+ medium.