Forty male albino rats were paired and given 15 trials in a dominance test situation where only one animal in a pair could eat at a given time. Length of time in control of the food and frequency of dominant gestures and failures, which involved either success or failure in displacing the other animal from the food source, were the criteria employed in determining the dominance relationship within a pair of animals. A second group of 20 animals, of the same age and strain, did not receive the competitive experience and served as a control group. The day following the termination of the dominance trials, an adult albino mouse was placed in the home cage of each rat in both groups. Results showed that: I. None of the rats in the non-competitive control group killed the mouse they were paired with. 2. Twenty-two of the rats in the competitive group killed. 3. Sixteen of the killer rats in the competitive group were dominant animals while only six were submissive. The results are explained as being due to the difference in past experience of the two groups of rats.