The laboratory rat is widely used in behavioral, physiological, and pharmacological studies of aggression. It is clear, however, that none of the tasks commonly used to measure aggression in the rat provide adequate examples of attack and defensive behavior. Introduction of strangers into established laboratory rat colonies produces a wide range of (often lethal) attack reactions in dominant colony animals and defensive behavior in the intruders. The proposed research program will attempt to determine the relationship between these behaviors and such tasks as reflexive fighting, isolation-induced aggression, tube-dominance, and mouse-killing, in order to evaluate an analysis of rat aggression based on the colony model. The comprehensiveness of this analysis will also be evaluated in terms of spontaneous colony fighting, and fighting in other domesticated and wild rat strains. These studies should therefore provide both a conceptual analysis of attack and defense in the rat, and a set of specific dependent variables for future research on the behavioral and physiological control of aggression.