Rac, a small GTPase in the ras superfamily, regulates at least two biological processes in animal cells: (i) the polymerization of actin and the assembly of integrin complexes to produce lamellipodia and ruffles; and (ii) the activity of an NADPH oxidase in phagocytic cells. NADPH oxidase activation is mediated through a rac effector protein, p67phox, and using chimeras made between rac and the closely related GTPase, rho, we have identified two distinct effector sites in rac, one N‐terminal and one C‐terminal, both of which are required for activation of p67phox. The same two effector sites are essential for rac‐induced actin polymerization in fibroblasts. p65PAK, a ubiquitous serine/threonine kinase, interacts with rac at both the N‐ and C‐terminal effector sites, but the GTPase‐activating protein, bcr interacts with rac at a different region. This makes p65PAK, but not bcr, a candidate effector of rac‐induced lamellipodium formation.