The effects of Anemonia sulcata toxin (ATX II) on action potentials and membrane currents were studied in single myocytes isolated from guinea-pig or bovine ventricles. Addition of ATX II (2-20 nM) prolonged the action potential duration without a significant change in resting membrane potential. Concentrations of 40 nM-ATX II or more induced after-depolarizations and triggered automaticity. The effects were reversible after washing or upon addition of 60 microM-tetrodotoxin (TTX). 5 mM-Ni did not modify the effects. The single patch-electrode voltage-clamp technique of Hamill, Marty, Neher, Sakmann & Sigworth (1981) was applied to record membrane currents in response to 8.4 S long depolarizations starting from a holding potential of -90 mV. Currents flowing later than 5 ms after the depolarizing step were analysed. The fast events could not be considered because of insufficient voltage homogeneity. After 2 min of exposure to ATX II (20 nM) the changes in net membrane currents were measured. The difference between the currents in the presence of ATX II and during control was defined as the 'ATX-II-induced current' (iATX). After 4 min of wash iATX disappeared. Within 10 S of exposure to 60 microM-TTX, iATX was blocked completely. At potentials positive to -60 mV, iATX was inwardly directed and decayed slowly but incompletely during the 8.4 S long depolarizing pulse. The rate of decay was faster during clamp pulses to more positive potentials. A high amplitude noise was superimposed on the current trace; its amplitude decreased with more positive potentials. We analysed the voltage dependence of iATX with 'isochronous' current-voltage relations. The 0.1 S isochrone of iATX was characterized by a 'threshold' for negative currents at -60 mV, a branch with a negative slope (k = -7 mV, potential of half-maximal activation (V0.5) = -38 mV, bovine cells) leading to a maximum inward current at -20 mV, and an ascending branch which led to an apparent reversal potential (Erev) around +40 mV. The values measured in guinea-pig myocytes were similar though not identical (k = -5.5 mV, V0.5 = -30 mV, maximum of inward current at -5 mV, Erev = +50 mV). Erev shifted to less positive potentials in later isochrones. Holding the membrane at -45 mV prevented the induction of extra current by ATX II. When the holding potential was then changed to -85 mV, iATX developed within some 2 min. Returning the holding potential to -45 mV blocked iATX with a similar slow time course.(ABSTRACT TRUNCATED AT 400 WORDS)