1. The mechanism of β adrenergic inhibition of pregnant rat myometrium was investigated. 2. Isoproterenol (4 x 10 -7 mol/l) caused membrane hyperpolarization of about 12 mV . The magnitude of this hyperpolarization was unaffected by K-free and Cl-free (isethionate substitution) solutions, and was reduced by about 50 % in the presence of ouabain (10 -3 mol/1) and at 10 °C. Hyperpolarization was calcium -dependent, was partially reduced by 12.5 mmol/1 [Ca 2+ ]0 and was abolished when 2.0 mmol/1 La was added to the bathing solution. 3. Isoproterenol (4 x 10 -7 mol/1) increased tissue cyclic AMP levels with a time course paralleling that of the relaxation both at 37 and at 10 °C. Papaverine (10 -4 mol/1) also showed similar actions. 4. Tissue calcium content as measured by the lanthanum technique increased during a K induced contracture and decreased when isoproterenol (4 x 10 -7 mol/1), papaverine (10 -4 mol/1), or dibutyryl cyclic AMP (10 -3 mol/1) relaxed the K contracture. 5. Another relaxant, D-600, a methoxy derivative of verapamil, (10-5 mol/1) had no effect on membrane polarization, tissue cyclic AMP or tissue calcium content. 6. It is concluded that there is an active component to the β adrenergic hyperpolarization of the rat myometium, although an electrogenic sodium pump is not likely to be involved. Rather an electrogenic calcium pump, possibly activated by cyclic AMP, is consistent with the data obtained. Moreover, a mechanism in which cyclic AMP stimulates calcium extrusion may underlie the adrenergically mediated relaxation in the myometrium .