The effects of ruthenium red (RR) on transmitter release and pre-synaptic currents were studied in the mouse neuromuscular junction. The action of RR (10 microM) was shown not only in the complete suppression of nerve-evoked muscle contractions associated with the depression of endplate potential amplitude but also in the partial inhibition of the amplitude of miniature-endplate potentials. However, the other ruthenium compounds, ruthenium chloride and tris (2,2-bipyridyl) ruthenium chloride did not significantly affect the neuromuscular transmission. In pre-synaptic waveform studies, the fast K(+)-current [IK(f)] as well as the ca(2+)-activated K(+)-current [IK(ca)] was significantly enhanced by 10 microM RR. Furthermore, 10 microM RR antagonized the action of beta-bungarotoxin (a blocker of slow K(+)-channel [IK(s)] in enhancing pre-synaptic Ca2+ currents. In contrast, the typical Ca(2+)-channel blockers, omega-agatoxin (0.5 microM), Gd3+ (0.5 mM) and CD2+ (0.3 mM) all suppressed the IK(ca). Although RR (1-30 microM) inhibited the Ca(2+)-currents of the nerve terminals induced by the combined treatment with the K(+)-channel blockers, 3,4-diaminopyridine plus tetraethylammonium chloride in a concentration-dependent manner, it is considered that RR-enhanced K+ currents were responsible for, at least in part, the observed inhibition of the Ca(2+)-current which led to the blockade of transmitter release.