Sex steroids exert profound influence on neural development and function through activation of intranuclear receptors. However, during sexual differentiation and at onset of puberty, intracerebral estrogen (E) availability is subsequent to these effects. The potent mitogen epidermal growth factor (EGF) activates estrogen receptor (ER)-dependent transcription in cultured cells in the absence of exogenous E. Since reproductive behavior in female rodents is the result of E-dependent transcriptional activity and protein synthesis, lordosis serves as a well established in vivo model for probing cellular and molecular mechanisms of steroid receptor-dependent behavior. Here we demonstrate that EGF can signal through the classical E receptor (ERalpha) to alter in vivo function in rodent central nervous system. EGF and EGF receptor ligands induced lordosis in a dose- and time-dependent manner in the absence of steroid treatment in ovariectomized rats and mice. Using antisense oligonucleotides, pharmacological and antibody blockade, and mutant mice, we also report that this behavioral responsiveness is mediated through ERalpha by specific stimulation of membrane-bound EGF receptors and EGF receptor-specific tyrosine kinase rather than by direct ligand activation of the ERalpha. Of biological significance, delayed onset of puberty and the absence of synchronization between reproductive behavior and ovulation was detected in intact mutant Wa-2 mice that express a naturally occurring point mutation in the EGF receptor. To our surprise, EGF-mediated behavior was independent of progesterone (P) and progesterone receptor (PR) since antiprogestins, PR antisense oligonucleotides, and targeted disruption of PR in ovariectomized transgenic mice failed to impede the display of lordosis after EGF. Finally, we also found that another growth factor, insulin-like growth factor-1, which provokes ER-dependent transcription in vitro, activates mating behavior in a similar E-independent manner. Thus, growth factor mediation of ER-targeted function may be a universal feature in the rodent central nervous system, raising critical questions about the role of growth factors in mediating ER-dependent processes in development and reproduction.