Slow action potentials were evoked in cruralis tonic and twitch fibres of the frog after drastically reducing the Cl- and K+ conductances. Tonic fibres were identified by their electrical characteristics. They had an effective resistance (Reff) of 50 +/- 6 M omega (n = 27) and a membrane time constant (tau m) of 440 +/- 70 ms (n = 8). In twitch fibres Reff = 2.9 +/- 0.3 M omega (n = 16) and tau m = 50 +/- 4 ms (n = 6). In tonic fibres the slow action potential had a threshold of -50 to -60 mV and a peak amplitude of -10 mV. In twitch fibres the slow action potential had a threshold of -40 mV and reached a peak amplitude of +40 mV. The responses were blocked by the addition of Cd2+ (2 mM) or Co2+ (5 mM). These results strongly suggest that Ca2+ is the main carrier of current during the response. Using the three-micro-electrode voltage-clamp technique a slow inward membrane current underlying the Ca2+ potential could be described in tonic muscle fibres. The slow inward current was mainly carried by Ca2+, since it was reduced when external Ca2+ concentration was lowered or when Cd2+ (2 mM) was added. Moreover, Ca2+ was the only cation in the solution that could carry inward current. It had a mean threshold of -60 mV, reached a maximum value at ca. 0 mV, ranged from 24 to 28 microA/cm2 and had a mean reversal potential of +35 mV. In about half of the examined tonic fibres inward current declined with time, only slowly. This can either be explained by there being less contamination by K+ outward current, or by the presence of two types of Ca2+ channels in the tonic fibre membrane.