Depolarizing action of 10-3 U/ml oxytocin and 10-4 g/ml carbachol on pregnant mouse myometrium in ionic solutions containing various concentrations of external Na and Ca was studied. 1) Excess Ca (10 mM) caused hyperpolarization in normal and in low Na (15.5 mM) solutions, and decreased membrane conductance. 2) The membrane was depolarized by about 10 mV in low Na solutions (15.5 mM) and membrane conductance was decreased. Reduction of external Ca to 0.25 mM caused further depolarization and an

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... increase membrane conductance. 3) Depolarization caused by carbachol was reduced in solutions containing low Na or excess Ca. The depolarizing action was restored by reducing external Ca to 0.25 mM in low Na solutions. The peak depolarization produced by oxytocin was little affected in any of the above ionic solutions. 4) In Na-free solutions, where the membrane was depolarized to -25 mV and contracture was produced, neither oxytocin nor carbachol was effective in producing changes in membrane potential or in tension development.