Gonadally-intact or castrated and testosterone-(T) treated male mice display aggressive behavior towards olfactory-bulbectomized male (OBM) stimuli, but not towards lactating female (LF) stimuli. By comparison, T-treated female mice display aggressive behavior towards both OBM and LF stimuli. The purpose of the present experiment was to determine if male mice given hyperphysiological T-treatment would display "female-typical" attack of OBM and LF stimuli. Hyperphysiological T-stimulation did not lead to the display of aggressive behavior towards OBM and LF stimuli; only OBM stimuli were attacked, suggesting a qualitative behavioral sex difference in response to T. However, the major finding of this study occurred following the termination of T-treatment. Castrated males that had previously received hyperphysiological T-treatment began to attack LF stimuli within 48 hr of treatment termination. By comparison, castrated males that had previously received physiological T-stimulation, as well as a gonadally-intact control group, generally began to attack LF stimuli 3-4 weeks following treatment-termination/castration. It is suggested that this unusual treatment-termination-induced behavioral display occurs via neurochemical mediation.