We have studied the effect of hypoxia and ouabain on the muscle resting potential (RP) of the cockroach Periplaneta americana L. The experiments were done in insects reared either in normoxic or hypoxic conditions and treated, or not treated, with ouabain. Hypoxic conditions decreased the RP by about 11.6 mV. Ouabain decreased the RP by about 8.3 mV (ouabain 10(-5) mol/l) or 12.3 mV (ouabain 5 x 10(-5) mol/l). Hypoxia and ouabain decreased the RP by about 28.5 mV (ouabain 10(-5) mol/l) or 31.3 mV (ouabain 5 x 10(-5) mol/l).