Hormone-dependent aggression in both male and female rats includes the distinctive behavioral characteristics of piloerection and lateral attack. In males the aggression is dependent on testicular testosterone and is commonly known as intermale aggression. In females, the aggression is most commonly observed as maternal aggression and is dependent on hormones whose identity is only beginning to emerge. The present review examines the experiential events which activate hormone-dependent aggression, the relation of the aggression to gonadal hormones, and the neural structures that participate in its modulation. In males and females, the aggression is activated by cohabitation with a conspecific of the opposite sex, by competitive experience, and by repeated exposure to unfamiliar conspecifics. In the female, the presence of pups also activates aggression. In both males and females, hormones are necessary for the full manifestation of the aggression. The essential hormone appears to be testosterone in males and a combination of testosterone and estradiol in females. The information available suggests the neural control systems for hormone-dependent aggression may be similar in males and females. It is argued that hormone-dependent aggression is behaviorally and biologically homologous in male and female rats.