1. Stretch-activated (SA) channels were studied in smooth muscle cells isolated from mesenteric resistance arteries using the whole-cell patch-clamp method. Membrane stretch was achieved by cell inflation after application of positive pressure through a patch electrode. 2. In the voltage-clamp configuration, cell inflation increased and cell deflation decreased the membrane conductance. Conductance of the evoked current depended on the increase in cross-sectional area of the cell. The current-voltage relationship was linear between -80 and 0 mV, while further hyperpolarization showed a slight inward rectification. 3. The reversal potential of the SA current depended on the extracellular Na+ concentration, suggesting that the inward SA current was carried predominantly by Na+. The SA current was also carried by other cations, suggesting that the channel responsible for this current is a non-selective cation channel. The permeability sequence of cations as assessed by reversal potential was as follows: K+ > or = CS+ > or = Na+ > Li+. The channel was also permeable to Ca2+. 4. Extracellular Ca2+ and Gd3+ inhibited the SA current carried by monovalent cations in a concentration-dependent manner with IC50 (concentration giving 50% of maximal inhibition) values of 0.9 mM and 14 microM, respectively. 5. In the current-clamp configuration, membrane stretch depolarized the cell, and 100 microM Gd3+ inhibited the stretch-induced depolarization. 6. The results suggest that SA cation channels exist in arterial smooth muscle cells. Activation of the channels may modify membrane potential and intracellular ionic environment, and promote stretch-mediated cell responses.