The steady-state and kinetic properties of Na(+) and K(+) currents of inner (white) and outer (red) muscles of zebrafish larvae 4-6 days post-fertilization (d.p.f.) are described. In inner muscle, the outward currents were half-activated at -1.0 mV and half-inactivated at -30.4 mV, and completely inactivated within 100 ms of depolarization. The inward currents of inner fibres were half-activated at -7.3 mV and half-inactivated at -74.5 mV and completely inactivated within 5 ms of depolarization. Inner muscle fibres were found to support action potentials, while no action potentials could be evoked in outer muscles. In inner muscle fibres, all tested levels of depolarizing current above a threshold value evoked only one action potential. However, spiking at frequencies of up to 200 cycles s(-1) was evoked by the injection of depolarizing pulses separated by short hyperpolarizing currents. We suggest that the properties of the inward sodium and outward potassium currents permit high frequency firing in response to a pulsatile depolarizing input of the kind expected in fast swimming, whilst safeguarding against tetany during a strong depolarization.