Inescapable electric shock was delivered to freely moving wild rats in the presence of familiar and novel inanimate targets. It was found that: (a) In response to shock, wild rats rarely attacked a familiar target but would readily attack a novel one. (b) Amount of escape-directed behavior in response to shock decreased as the result of introducing a novel target into the shock situation. (c) In a choice situation, both proportion of attacks directed toward a novel target, as compared with a familiar one, and number of attacks directed toward the novel target decreased with increased exposure to the novel target. Azrin, Hutchinson, and Sallery (1964) have described a procedure by which domesticated rats can be induced to bite an inanimate target in response to unavoidable shock. Rats in this Azrin et al. (1964) study were restrained Y2 in. from a target obj ect and received intense electric shock (5 ma.) to the tail. The shock resulted in biting of the target on almost every trial. Unfortunately, this procedure, which permits objective recording of the number and duration of bites, is of somewhat limited usefulness in the investigation of conditions eliciting aggressive behavior. Because of the confinement necessary to produce biting, the rats are severely limited in their response repertoire and the experimenter is limited in his choice of independent variables. Wild rats are inherently far more aggressive in a variety of situations than their domesticated conspecifics (Barnett, 1958, 1963; Galef, in press; Karli, 1956; Richter, 1949; Stone, 1932). Preliminary investigation revealed that a single footshock of 1.3ma. intensity and 1.0-sec. duration delivered to a freely moving wild rat could elicit as many as 19 discrete biting episodes directed toward an inanimate object in the 1