The present study was undertaken to investigate the postsynaptic effects of aminoglycosides on contractions evoked by acetylcholine (ACh), KCl, electrical field stimulation (EFS) and Na+-and Ca2+-free Ringer solution with 0.2 mM Na-2 EDTA (NaFCaFR) in the isolated frog rectus abdominis. Neomycin inhibited contraction elicited by ACh, NaFCaFR, and EFS at the higher frequencies (8 and 10 Hz) but not those elicited by KCI and EFS at the lower frequencies (2, 3 and 5 Hz). D-tubocurarine inhibited ACh-induced contractions in a concentration-dependent manner. In addition, drug reduced EFS-evoked contractions to a limited extent. Lower concentrations (10(-5), 5x10(-5), 10(-4), 2x10(-4) and 3 x 10(-4) M) but not higher concentrations (4 x 10(-4) and 5 x 10(-4) M) of methoxyverapamil exhibited a concentration-dependent inhibitory action on Na, Ca, R-induced contractions. Similar inhibitions of the same type of contraction were displayed by aminoglycosides (neomycin, streptomycin, netilmycin, gentamycin and amikacin). These results suggest that in addition to their antagonistic action on nicotinic receptors in the frog rectus abdominis, aminoglycosides may exert stabilizing effects on some functional components contributing to contractions at the membrane.