The purpose of the present study was to investigate α1-adrenergic receptors in the heart as well as the activity and the sensitivity of the phosphoinositide pathway on tissue slices of atria, ventricles, and femoral artery of hypertensive rats treated for 4 weeks with deoxycorticosterone acetate (DOCA) and 1% saline. DOCA-salt hypertensive rats were characterized by an increased sympathoadrenal tone, as suggested by increased norepinephrine and epinephrine plasma levels. The basal activity of the phosphoinositide pathway, estimated by measuring the accumulation of inositol monophosphate in the presence of an excess of lithium, was found to be greater in atria than in ventricles and femoral artery in both normotensive and DOCA-salt hypertensive rats, but it was twofold greater in atria and ventricles ofDOCA-salt hypertensive rats compared with normotensive rats. Following stimulation by norepinephrine, the production of inositol monophosphate was greater in atria and femoral artery than in ventrides in both groups. However, in DOCA-salt hypertensive rats, the production of inositol monophosphate was markedly enhanced, being about twofold greater in atria and femoral artery and about three times greater hi ventricles than hi tissues of normotensive rats. These differences between DOCA-salt hypertensive and normotensive animate do not appear to be associated with a difference in α-adrenergfc receptor number or affinity since cardiac α1-adrenergic receptor number was unchanged hi hypertensive rats and the binding affinity to the receptor was significantly decreased in hypertensive rats compared with normotensive rats. These data therefore demonstrate an increase In basal and norepinephrine-lnduced inositol monophosphate production in cardiovascular tissues of DOCA-salt hypertensive rats and suggest the existence of a hypersensitlvity of α1-adrenergic receptors or of an increased sensitivity of intracellular mechanisms responsible for the activation of the phosphatidylinositol pathway in the cardiovascular tissues of these animals.