1. Membrane currents of isolated smooth muscle cells from rat portal vein were studied using the whole-cell patch-clamp technique. 2. In addition to the Ca(2+)-dependent Cl- current, single-channel activities were recorded in response to external applications of 10 mM-caffeine, in the absence of EGTA in the pipette solution. The conductance of this novel type of channel was around 200 pS for membrane potentials ranging from -100 to +60 mV. 3. The single-channel activities were also induced by external application of noradrenaline (10(-5)M) or acetylcholine (10(-5)M), by Ca2+ entry through voltage-dependent Ca2+ channels, and by intracellular application of ryanodine (10(-5)M). The caffeine-activated single-channel currents disappeared when 10 mM-EGTA was added to the pipette solution or after replacement of external Ca2+ with Ba2+. These results show that these channels are Ca2+ dependent. 4. Alterations of the Cl- equilibrium potential did not produce any change in the reversal potential of the caffeine-activated single-channel current indicating that it was not carried by Cl- ions. The value of the reversal potential was about +10 mV, irrespective of the CsCl-, KCl- or NaCl-containing solutions used to fill the pipette. This observation indicates that the channel was equally permeable to these monovalent cations. 5. Caffeine activated single-channel currents when cells were bathed in 90 mM-Ba2+ plus 1 mM-Ca(2+)- or 91 mM-Ca(2+)-containing solutions, showing that divalent cations permeate the channels. The permeability for Ca2+ over Na+ was high as the ratio PCa/PNa was estimated to be 21. 6. Single-channel activities induced by caffeine were not modified by Ca2+ channel blockers such as dihydropyridines and phenylalkylamines. 7. It is concluded that portal vein smooth muscle cells possess Ca(2+)-activated nonspecific channels which are highly permeable to Ca2+ ions.