1. Whole-cell membrane current recordings under voltage clamp were made at room temperature from dispersed single cells of longitudinal smooth muscle of rabbit jejunum and dispersed single smooth muscle cells of rabbit ear artery using patch pipettes containing up to 10 mM-EGTA Ca buffer. 2. Spontaneous transient outward currents (s.t.o.c.s) up to 250 pA in size and about 100 ms in duration were observed in conditions which might lead to an elevated internal Ca concentration. The amplitude distribution in some cells and form of the currents suggested that they were evoked by a quantal stimulus. 3. S.t.o.c. amplitude was voltage dependent and reversed at the K equilibrium potential. S.t.o.c.s. were blocked by 1 mM-tetraethylammonium or 10 mM-Ba applied externally or by perfusing Cs inside the cell. 4. Removing external Ca abolished s.t.o.c. activity in the jejunal cells but not in arterial cells. Increasing EGTA buffering within the cells from 1 mM or less to 10 mM abolished activity in both cell types. 5. Caffeine (5 mM) applied to the bathing solution stimulated rapid discharge of transient outward currents and then a prolonged period of inhibition. The stimulated discharge was sensitive to external Ca in jejunal cells but much less so in arterial cells. ACh applied by ionophoresis to jejunal cells or noradrenaline bath-applied onto arterial cells also stimulated discharge of transients followed by a prolonged inhibitory phase. 6. It was suggested that s.t.o.c.s represent the simultaneous opening of up to 75-100 Ca-activated K channels at -40 mV in response to sudden discharge of Ca from internal stores when these became overloaded, and that this process may occur cyclically.