Several proteins present on the sperm membranes serve as signal transducers facilitating sperm maturation, capacitation, acrosome reaction and fertilization. Among them, guanylate cyclase (GC), the inositol trisphosphate receptor (IP 3 R) and calmodulin have been reported rather extensively. Although biochemical data clearly suggest their roles in the fertilization process, the precise location of these proteins in the sperm membranes have not been adequately investigated and need clarification. Using immunolocalization procedures we report that guanylate cyclase receptors (GCR) are localized on the apical region of the acrosome and on the postacrosomal plasmalemma, while IP 3 Rs are present in the neck region of the sperm and at lower density along the axonemal membrane. In contrast, calmodulin is restricted to the postacrosomal, basal nuclear membrane and the neck region that corresponds to the redundant nuclear envelope and ER vesicles, with some reaction along the middle piece and tail membranes. Colocalization of these proteins on the sperm membrane together with biochemical data as reported previously provide further insight into their role in the spermatozoan motility and possibly the fertilization process.