1. The effect of exogenous ATP on transmembrane currents and on the cytoplasmic Ca2+ has been investigated in single cultured smooth muscle cells of pig aorta. 2. ATP applied to cells held at a potential of -50 mV evoked a transient inward current and a transient rise in [Ca2+]i. At a potential of +20 mV the ATP-induced increase in [Ca2+]i was accompanied by an outward current. 3. At a potential of -50 mV, ATP evoked in Ca(2+)-free solution an inward current which was similar to that in the presence of external Ca2+. A second application of ATP in Ca(2+)-free solution induced a much smaller current. 4. ATP induced in Ca(2+)-free solution a pronounced transient stimulation of the 45Ca2+ efflux from confluent smooth muscle monolayers. 5. The I-V curve of the ATP-activated current has a reversal potential close to 0 mV. A reduction of external Cl- shifts this reversal potential in accordance with the change of the Cl- equilibrium potential. 6. It is concluded that ATP causes a release of calcium from intracellular stores. The ensuing increase of [Ca2+]i activates a Cl- current, which can depolarize the cell membrane and thereby promote a voltage-gated Ca2+ entry.