The mechanisms by which testosterone regulates aggression are unclear and may involve changes that alter the activity levels of one or more brain nuclei. We estimate neural activity by counting immunopositive cells against phosphorylated cyclic AMP response element binding protein (pCREB). We demonstrate increased pCREB immunoreactivity within the dorsolateral subdivision of the ventromedial hypothalamus (VMHdl) following an aggressive encounter in male tree lizards Urosaurus ornatus. This immunoreactivity is induced both by exposure to and performance of aggressive behaviors. This dual activation of the VMHdl suggests its possible role as an integration center for assessment and expression of aggressive behavior. Furthermore, pCREB induction was greater in encounters involving higher frequency and intensity of aggressive display, demonstrating a direct relationship between neural activation and behavior. The VMHdl is also rich in steroid receptors. In a second experiment involving hormone manipulations, testosterone treatment increased aggression levels, though it did not increase the number of pCREB positive cells within the VMHdl. This lack of an effect of testosterone on pCREB induction within the VMHdl may be due to induction arising from the behaviors of conspecifics (especially in low-testosterone, low-aggression individuals), variation in aggression mediated by other variables, or regulation of aggression by circuits outside of the VMHdl. Together, these findings support a notion of the VMHdl as a nucleus involved in integrating afferent and efferent information within the neural aggression-control circuit.