The deep lateral muscle fibres of the trout (Salmo irideus) have a polyaxonal and distributed motor innervation. Electrical activity was investigated using intracellular microelectrodes under current clamp conditions. In high-potassium solutions, the resting membrane potential depended on the external potassium concentration. However low-potassium or low-chloride solutions produced steady-state membrane depolarizations. In such solutions, a large decrease in the external sodium concentration slightly repolarized the membrane. Inward and outward rectifications could be observed. Only the outward rectification was decreased by tetraethylammonium ions (20 mM). In high-sodium solutions, the application of depolarizing currents induced an autoregenerative electrical activity which was suppressed by tetrodotoxin (10 microM). Tetraethylammonium ions (20 mM) increased both amplitude and duration of the action potential. It is concluded that the membrane of the muscle fibres has several electrical properties similar to those of twitch skeletal fibres of vertebrates. The increase in motor nerve stimulation induced both junctional potentials and spike-like responses. The latter had a variable amplitude and rarely reached the zero potential.