The postsynaptic effects of two aminoglycoside antibiotics, streptomycin and neomycin, were studied on miniature end-plate currents (mepcs) and acetylcholine-induced end-plate current fluctuations in voltage-clamped costocutaneous muscles of the garter snake (species Thamnophis). Neomycin decreased the amplitude of mepcs and accelerated the time constants of mepc decay in a concentration-dependent manner without altering the single exponential nature of mepc decay. Neomycin also produced a voltage- and concentration-dependent nonlinearity in the current/voltage relationship. The relationship between the time constants of mepc decay and membrane potential was progressively reduced with increasing concentrations of neomycin. A concentration-dependent reduction in single channel conductance and channel lifetime was also obtained with neomycin. In contrast, streptomycin, in concentrations up to 5 X 10(-5) M, did not significantly alter either mepc amplitude, the time constant of mepc decay, the relationship between the mepc decay time constant and membrane potential or the lifetime and conductance of single end-plate channels. In very high concentrations (greater than 1 mM) streptomycin decreased mepc amplitude and prolonged mepc decay at hyperpolarized membrane potentials. The results suggest that neomycin interacts with the ionic channels of the acetylcholine receptor in their open configuration, whereas streptomycin acts primarily by blocking the receptor. The significant differences in the molecular actions of these two antibiotics may provide an explanation for the observed differences in the character and reversal of the neuromuscular block produced by these antibiotics.