The aggressive behavior of alpha male rats and lactating females were each examined toward an intact adult male rat, a castrated adult male rat, an anesthetized adult male rat, a nonlactating adult female rat, an adult albino guinea pig (male or female), or an albino mouse (male or female). When in their living colony, females displayed high levels of aggressiveness toward all stimulus objects except a mouse. The aggression toward the intruding males occurred whether the female's pups were present or not. Alpha males were aggressive toward the same stimuli except an intruding female rat and a mouse. When tested in an unfamiliar colony, the males but not the females (with or without pups present) were aggressive toward an adult male rat. Half of the females but none of the males displayed defensive burying toward an anesthetized intruder. It is suggested that the attack on an adult female, the absence of attack outside of the resident colony, and the tendency to display defensive burying are features of the aggressiveness of lactating females that are fundamentally different from the aggressiveness of alpha males. The form of the aggression (lateral attack vs. lunge attack) was only quantitatively different in males and females.