The action of gallamine, a classical competitive neuromuscular blocking agent, has been examined on voltage-clamped endplates of frog skeletal muscle fibres. Gallamine produces a parallel shift of the equilibrium log (concentration)--response curves in concentrations of up to about 40 microM. At a membrane potential of -70 mV the Schild plot of the dose ratios so measured has a gradient of slightly less than the theoretical value, for a competitive antagonist, of unity. The apparent equilibrium constant for 'competitive' block is about 2 microM, and is approximately independent of the membrane potential. Fluctuation analysis of the endplate current shows two components in the presence of gallamine. The results can be fitted, over the range tested, by a mechanism that involves block of open ion channels by gallamine in a manner similar to that by procaine or quaternary local anaesthetic analogues. The rate constants for this action are strongly dependent on the membrane potential. At -100 mV the association rate constant is about 4 x 10(7) M-1S-1, the dissociation rate constant is about 600 s-1, and the equilibrium constant about 15 microM. Other kinetic measurements (voltage-jump relaxation, and nerve-evoked endplate currents) give results consistent with this conclusion, but apparently these results are valid over a range of conditions narrower than that for fluctuation analysis.