Various receptors coupled to the heterotrimeric guanine nucleotide-binding protein Gq/11 stimulate formation of inositol-1,4,5-trisphosphate (IP₃). Activation of these receptors also induces protein tyrosine phosphorylation. Formation of IP₃ in response to stimulated receptors that couple to Gq/11 was blocked by protein tyrosine kinase inhibitors. These inhibitors appeared to act before activation of Gq/11. Moreover, stimulation of receptors coupled to Gq/11 induced phosphorylation on a tyrosine residue (Tyr³⁵⁶) of the Gα_q/11subunit, and this tyrosine phosphorylation event was essential for Gq/11 activation. Tyrosine phosphorylation of Gα_q/11 induced changes in its interaction with receptors. Therefore, tyrosine phosphorylation of Gα_q/11appears to regulate the activation of Gq/11 protein.