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Barbiturates alter cardiovascular function, in part by an effect on vascular cells. However, a biochemical mechanism for the effect is unknown. We have, therefore, studied the effect of barbiturates on inositol phospholipid hydrolysis in cultured rat aortic endothelial cells. Hydrolysis was stimulated by angiotensin II, norepinephrine and phenylephrine. Pentobarbital, and other barbiturates, inhibited hydrolysis at pharmacological and clinical concentrations (0.1–0.5 mM). The inhibition by pentobarbital was concentration-dependent, reversed by washing, and was decreased by high concentrations of angiotensin II. Kinetic studies gave an apparent K m of hydrolysis by angiotensin II of 1.2 nM, which showed mixed inhibition by pentobarbital (K i = 0.45 mM). Schild analysis of data obtained from pentobarbital inhibition curves also showed a deviation from a competitive type inhibition. [ 125 I]Angiotensin II was bound to a high-affinity receptor (K d = 1.2 mM), which showed a competitive type inhibition of binding by pentobarbital (0.5 mM). Although inhibition of [ 125 I]angiotensin II binding appeared to be competitively inhibited by pentobarbital, the data, taken together, point to a deviation from a simple competitive type inhibition.