Receptor-stimulated phosphatidylinositol turnover has been studied in isolated, perfused, [3H]inositol-labelled rat hearts by measuring accumulation of inositol phosphates in the presence of lithium chloride. Inositol phosphate accumulation was stimulated by norepinephrine (3±10-5M) and carbachol (10-3M), the increases averaging from 931 ± 59 (n = 6, mean ± SEM, cpm/g heart) to 4,165 ± 609 (n = 6, p<0.01) for norepinephrine and to 1,853 ± 354 (n = 6, p<0.05) for carbachol. The norepinephrine stimulation was antagonized by prazosin (10-7M) but not by propranolol (10-7M), indicating mediation via α1-adrenoceptors. The carbachol stimulation was antagonized by atropine (10-7M). The stimulation by norepinephrine was significantly higher in right atria (837 ± 151 to 6,614 ± 1,210, n = 6, cpm/g tissue) than in other regions of the heart. Both norepinephrine and carbachol stimulated the formation of inositol monophosphate, inositol bisphosphate, and inositol trisphosphate with norepinephrine stimulation being detected as early as 15 seconds. Furthermore, the inositol trisphosphate was identified as the -1,4,5 isomer by anlon exchange high-performance liquid chromatography. These data are consistent with the hydrolysis of phosphatidylinositol-(4,5)-bisphosphate yielding inositol-(l,4,5)-trisphosphate. Inositol-(l,3,4)-trisphosphate was not detected in heart preparations, suggesting unusual metabolism of inositol-(l,4,5)-trisphosphate in heart tissue. (Circulation Research 1987;61:625-631)