Fibres isolated from the ventral sucker of Fasciola hepatica were identified as muscle on the basis of their contractility, and their actin and myosin staining. They were voltage-clamped at a holding potential of -40 mV and depolarization-activated outward currents were characterized both electrophysiologically and pharmacologically. Activation was well fitted by a Boltzmann equation with a half-maximal potential of + 9 mV and a slope factor of -14.3 mV, and the kinetics of activation and deactivation were voltage-sensitive. Tail current analysis showed that the reversal potential was shifted by +46+/-3 mV when E(K) was increased by 52 mV, confirming that this was a K+-current with electrophysiological characteristics similar to delayed rectifier and Ca2+-activated K+-currents in other tissues. The peak current at + 60 mV was inhibited by 76+/-6% by tetrapentylammonium chloride (1 mM) and by 84+/-7% by Ba2+ (3 mM), but was completely resistant to block by tetraethylammonium (30 mM), 3,4-diaminopyridine (100 microM) and 4-aminopyridine (10 mM). Penitrem A, a blocker of high-conductance Ca2+-activated K+-channels reduced the current at +60 mV by 23+/-5%. When the effects of Ca2+-channel blocking agents were tested, the peak outward current at + 60 mV was reduced by 71+/-7% by verapamil (30 microM) and by 59+/-4% by nimodipine (30 microM). Superfusion with BAPTA-AM (50 microM), which is hydrolysed intracellularly to release the Ca2+-buffer BAPTA, also decreased the current by 44+/-16%. We conclude that voltage-and Ca2+-sensitive K+-channels are expressed in this tissue, but that their pharmacology differs considerably from equivalent channels in other phyla.