THE Ras-related Rho subfamily of GTP-binding proteins (p21s), which includes Rho, Rac and Cdc42Hs, is implicated in different aspects of cytoskeletal organization^1,2. These proteins behave like Ras (p21^ras) in that their active GTP-bound form is inactivated by intrinsic hydrolysis of the nucleotide γ-phosphate, which can be stimulated by GTPase-activating proteins (GAPs). We have previously shown that there is a diversity of GAPs that recognize this subfamily³, including n-chimaerin, which is enriched in the hippocampus⁴; we also detected proteins that bind these p21 proteins and seem to inhibit GTP hydrolysis. We now report the characterization of a hippocampal complementary DNA encoding a tyrosine kinase that specifically binds Cdc42Hs in its GTP-bound form. This binding is mediated by a unique sequence of 47 amino acids C-terminal to an SH3 domain and inhibits both the intrinsic and GAP-stimulated GTPase activity of Cdc42Hs. Our findings indicate that there may be a regulatory mechanism that sustains the GTP-bound active form of Cdc42Hs and which is directly linked to a tyrosine phosphorylation pathway.