We have measured the effects of morphine on the intracellular electrophysiological properties of smooth muscle cells from the circular and longitudinal muscle layers of the canine jejunum. Morphine hyperpolarized the circular muscle membrane by approximately 12 mV and increased the electrical control activity (ECA) amplitude and dV/dt. Morphine had no significant effect on the electrical properties of the longitudinal muscle cells. The morphine-induced hyperpolarization of the circular muscle membrane was blocked by tetrodotoxin (TTX) and naloxone, but not by atropine and hexamethonium, propranolol, or phentolamine. Morphine significantly increased the slope of the resting membrane potential vs. the log of the potassium concentration in bathing medium from 38 to 50 mV/decade. The sodium permeability to potassium permeability ratio, calculated from the Goldman constant field equation, was reduced by morphine from 0.13 to 0.07 at mM of K+. The above results suggest that when measured by intracellular techniques, morphine hyperpolarizes the circular muscle membrane by release of a nonadrenergic, noncholinergic neurotransmitter. The mechanism of this hyperpolarization is consistent with an increase in potassium conductance.