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The effect of histamine on [3H]-inositol phosphates (IPs) formation was investigated with [3H]-inositol-labeled gastric smooth-muscle cells in guinea pigs. Histamine (10-5 M) increased the formation of [3H]-IPs in the muscle cells. The increase was significantly inhibited by pyrilamine (10-5 M) but not by cimetidine (10-5 M). The contractile response to histamine was also completely inhibited by pyrilamine but not by cimetidine. Phorbol ester 12-myristate 13-acetate (10 μM) significantly inhibited histamine-stimulated [3H]-IPs formation by 56%, whereas forskolin (10 μM) decreased it by 18%. This study demonstrates that the response of [3H]-IPs formation and contraction to histamine is mediated through H1 receptor, and the formation of [3H]-IPs is negatively regulated by protein kinase C in gastric smooth muscle of guinea pigs.