Abstract: Phosphoinositide turnover stimulated by 5‐hy‐droxytryptamine (5‐HT) receptors in the intact rat brain was studied using an in vivo method. Phosphoinositides in the rat brain were prelabeled with [3H]inositol injected into the lateral cerebral ventricles. The rats were killed by microwave irradiation after 48 h and the contents in the frontal cortex of 3H‐inositol phosphates, [3H]inositol‐1‐monophosphate ([3H]IP1), [3H]inositol‐l,4‐bisphosphate ([3H]IP2), and a mixture of [3H]inositol‐l,4,5‐trisphosphate and [3H]inositol‐1,3,4‐trisphosphate ([3H]IP3) were assayed by HPLC. Lithium treatment (10 mEq/kg, i.p., 2 h before) increased the content of[3H]IP1 and[3H]IP2. 5‐Methoxy‐N′,N‐dimethyltryptamine (5‐MeODMT) and quipazine, 5‐HT agonists, significantly increased the amount of 3H‐inositol phosphates under lithium pretreatment. The response to 5‐MeODMT was inhibited by ritanserin, a 5‐HT2 antagonist, but not by (—)‐propranoloI, a 5‐HT1 antagonist. These results suggest that phosphoinositide turnover in the rat frontal cortex in vivo is stimulated by 5‐HT2 receptor activation. It is considered that this method will be useful for measurement of 5‐HT2 receptor‐stimulated phosphoinositide turnover in vivo to examine the in vivo effects of various psychotropic drugs such as antidepressants. Copyright © 1989, Wiley Blackwell. All rights reserved