We studied acutely dissociated neurons from the dorsomedial (shell) region of the rat suprachiasmatic nucleus (SCN) with the aim of determining the ionic conductances that underlie spontaneous firing. Most isolated neurons were spontaneously active, firing rhythmically at an average frequency of 8 ± 4 Hz. After application of TTX, oscillatory activity generally continued, but more slowly and at more depolarized voltages; these oscillations were usually blocked by 2 μm nimodipine. To quantify the ionic currents underlying normal spontaneous activity, we voltage clamped cells using a segment of the spontaneous activity of each cell as voltage command and then used ionic substitution and selective blockers to isolate individual currents. TTX-sensitive sodium current flowed throughout the interspike interval, averaging -3 pA at -60 mV and -11 pA at -55 mV. Calcium current during the interspike interval was, on average, fourfold smaller. Except immediately before spikes, calcium current was outweighed by calcium-activated potassium current, and in current clamp, nimodipine usually depolarized cells and slowed firing only slightly (average, ∼8%). Thus, calcium current plays only a minor role in pacemaking of dissociated SCN neurons, although it can drive oscillatory activity with TTX present. During normal pacemaking, the early phase of spontaneous depolarization (-85 to -60 mV) is attributable mainly to background conductance; cells have relatively depolarized resting potentials (with firing stopped by TTX and nimodipine) of -55 to -50 mV, although input resistance is high (9.5 ± 4.1 GΩ). During the later phase of pacemaking (positive to -60 mV), TTX-sensitive sodium current is dominant.