1. Abrupt warming induces a transitory hyperpolarization across a sucrose gap in ventricular myocardium of Aplysia californica. 2. Abrupt cooling induces a transitory depolarization. 3. The effects of a sudden change of temperature are reversed in bathing medium lacking potassium. 4. Ouabain counteracts the depolarization induced by cooling. 5. Stepwise reduction in bath potassium has a stepwise depolarizing excitatory effect. 6. Acetylcholine treatment or reduction in bath sodium counteracts the depolarizing effect of reduction in bath potassium. 7. The effects of warming and cooling, which are counteracted by ouabain or potassium deprivation, as well as the depolarizing effects of potassium deprivation, support the presence of an electrogenic sodium pump. The action of acetylcholine in counteracting depolarization is congruent with its hyperpolarizing effect on this muscle.