Agonistic behavior of rats of the Wistar and WEzob strain was compared. Two male rats of the same strain or two male-female pairs of the same strain were confronted with each other for a period of one hour. Individuals and pairs were unknown to each other. In the week before the pairs or the single males were placed in the two compartments of a box. By removing a separating panel, encounters could take place. Behavior was videotaped and analyzed afterwards. During the confrontation session,

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... s from pairs displayed far more consummatory acts (clinch fights and attacks, Le., biting or attempting to bite) than the individual housed males. The individually housed males, however, showed overall longer agonistic behavior, but this consisted of mainly appetitive agonistic behavior such as lateral attack, keeping down, keep off lying, them males from pairs. There were striking differences between strains: Wistar males from pairs showed more frequent and longer clinch fights and attacks than those of the WEzob strain. The former got far more wounds. It seems that belonging to the losing strain in interstrain encounters is not a good predictor of the amount of intrastrain aggression. The WEzob females displayed more frequent and longer appetitive behavior than the Wistar females. In male-female interactions the WEzob rats displayed more fi-equently some appetitive behaviors than the Wistar rats, but the Wistar rats displayed more and longer clinch fights than the WEzob rats. The enhancement of male aggression by the presence of a female seems to be a strain -dependent phenomenon. Usually male rats display agonistic behavior against intruders in their residence, especially if this intruder is a male. This agonistic behavior of the resident male is labelled as territorial behavior, Le., behavior to retain territory. Rats trying to extend their territory at the cost of neighbouring rats also display sigonistic behavior (Calhoun, 1962; Moyer, 1971 ). The resident rat is usually the winner. This phenomenon is known as the prior residence or ownership effect (Waser and Wiley, 1979) . If the loser cannot flee, as is common in experimental situations, a dominant-submissive relation is established. Social isolation enhances agonistic behavior of male rats (Blanchard & Blanchard, 1979) and so does the presence of a female or her odor in the cage (Flannely & Lore, 1977). In interstrain encounters WEzob rats (a Dutch strain) are defeated by S3 rats (Tryon Maze dull rats) but win over Wistar rats (Van de