In many smooth muscles a monovalent cation selective current provides a major excitatory input during muscarinic receptor stimulation. Whole-cell current shows conspicuous voltage-dependence (Benham et al., 1985) but its single channel properties are not known.Experiments were performed on single enzyme-dispersed myocytes from guinea-pig small intestine (Zholos & Bolton, 1994) of adult male guinea-pigs killed according to Schedule 1 of the Animals (Scientific Procedures) Act (1986). Cationic channels were activated in outside-out (n=123) or cell-attached patches (n=89) using either bath carbachol (0.05 mM) or internal GTPγS (0.2 mM) application (symmetrical 124 mM Cs⁺ solutions were used; intracellular Ca²⁺ was clamped at 10^-7 M by the use of BAPTA/CaCl₂ mixture). A cationic channel of 57.1±7.0 pS (means±S.E.M., n=52) conductance showed marked voltage-dependent behaviour. Its open probability (Po) increased with membrane depolarization e-fold per 19.9±4.0 mV to reach a maximal value of 0.81±0.09 with a potential of half-maximal activation of −83.9±10.7 mV (n=4) (properties closely similar to those of the whole-cell current; Zholos & Bolton, 1994). Moreover, ensemble averaged patch current mimicked both the whole-cell current-voltage relationship and current relaxations during negative voltage jumps.Single-channel patches generating about 10,000 of events revealed channel transitions between four closed-and-open state pairs. Analysis of adjacent state dwell times showed a strong inverse relationship between mean closed and open times, thus (closed/open in ms) 0.7/190, 4.7/47, 59/5.4 and 471/0.5, respectively (n=11). Depolarization reduced shut and prolonged open dwell times in each pair but has little or no effect on the relative contribution to total current of the four closed/open pairs identifiable within the frequencies which could be resolved (i.e., below 2 kHz). The longest open state of 190±43 ms mean dwell time (n=11) was the major determinant of Po and thus generated most of the whole-cell current (ca. 73 %). It arises most likely through interaction of the activated G protein with the channel but further work is needed to confirm this.