Single myelinated nerve fibres of Rana esculenta were investigated under current and potential clamp conditions at 20 degrees C. Under 2.5 mM phenobarbital, the amplitude of the action potential was reversibly reduced to 85.5 +/- 5% (n = 6), and the threshold potential was raised by 32% of the control in Ringer solution. The resting potential remained constant. Solutions with 2.5 mM phenobarbital caused a decrease of the Na current to 38.3 +/- 5.6% (n = 10), when the fibre had its holding potential before the test step. The effect was reversible at wash out of the drug. The Na currents were only negligibly decreased (7 +/- 4.5%; n = 10) when the test pulse was preceded by a long lasting negative polarization to Em = -140 mV. The effect of the conditioning polarization could be described by two time constants, tau 1 = 7.1 +/- 2.0 msec and tau 2 = 44.5 +/- 9.5 msec (n = 5). Experiments with 500 msec conditioning pulses showed that the Na inactivation curve, h infinity(Em), was shifted in a negative direction along the potential axis. In the range between 0.5 and 5.0 mM phenobarbital there was a shift of 8 mV for an e-fold change in drug concentration. 15 mM Ca2+ caused a shift to the h infinity(Em) curve in a positive direction along the potential axis, while simultaneous application of 2.5 mM phenobarbital and 15 mM Ca2+ caused no shift of the h afinity(Em) curve. The undissociated drug seemed to be responsible for the effects (pK = 7.3).