1. Stretch-activated channels (SACs) were analysed on patches attached to myocytes isolated from the guinea-pig urinary bladder. At 22 degrees C application of -2 to -4 kPa to the patch electrode induced SACs at a density of one to two per patch (3-5 M omega electrodes). 2. With electrodes containing 145 mM K+, 20 mM TEA and 2 mM Mg2+, the single channel current followed a linear I-V curve with a slope conductance of 39 +/- 5 pS (mean +/- S.D.) and a reversal potential of 2 +/- 6 mV. Substitution of chloride by aspartate ions left both parameters unchanged suggesting that the anions do not contribute to the currents. 3. Hyperpolarization from -30 to -80 mV did not open channels by itself but increased channel activity (NPo; where N is the number of channels in the patch and Po is the probability of the channel being open) twofold. The hyperpolarization-induced increase in NPo can be attributed to a reduction of long closures. At positive patch potentials numerous blank records strongly diminished NPo. 4. Inward currents through SACs can be carried by a variety of cations. In the presence of 2 mM Mg2+, the respective channel conductance was 40 +/- 4 pS for 140 mM K+ > 34 +/- 2 pS for 140 mM Na+ > or = 33 +/- 6 pS for 140 mM Cs+ > 19 +/- 2 pS for 110 mM Ba2+ > 17 +/- 2 pS for 110 mM Ca2+. 5. Reduction of CaCl2 from 110 to 10 mM did not change the conductance but shifted the reversal potential from +7 to -7 mV; the reversal potentials suggest that SACs are slightly more permeable for Ca2+ than for K+. 6. In the absence of divalent cations, the conductance of K+ was 82 +/- 4 pS for inward but 45 pS for outward currents. Addition of either 2 mM Ca2+ or 2 mM Mg2+ reduced the conductance for inward currents to 40 pS. 7. The change from 140 to 14 mM KCl plus 136 mM Tris-Cl reduced the conductance from 82 to 56 pS whereas the reversal potential shifted only from -4 to -9 mV. When 20 mM K+ and 300 mM sucrose were applied, the conductance fell to 39 pS and the reversal potential shifted by -30 mV. The results suggest that Tris+ can permeate through SACs when extracellular divalent cations are absent.(ABSTRACT TRUNCATED AT 400 WORDS)