A method was developed for producing and measuring territorial aggression in male CF-1 laboratory mice using a simple apparatus. The technique is based on data collected from approximately 1000 CF-1 mice in order to establish the parameters and optimize the procedures. In this technique the mouse takes up lone residence for 24 hr in a 60 cm square box attached by a tubular runway to a standard mouse (home) cage with food, water, and bedding. After this interval, a naive intruder male CF-1 mouse of the same age is introduced. Under control (no treatment) conditions, 85-90 percent of the resident mice will attack the intruder with a latency of about 5 min and all residents attacking the intruder are dominant. Dominance or submission is typically decided within the first 20 min of the test. Data on 10 pairs of mice can be collected simultaneously by one observer. Treatments can be assessed in terms of their effects on the production of aggression (percentage of animals attacking) in either the resident or the intruder, and on the level of aggression produced by recording the latency to attack, the frequency of attacks, and the number of animals wounded (showing blood) during the 20 min observation period. The advantages of the technique include the use of a naturally occurring aggressive behavior (as opposed to techniques empolying long periods of isolation, shock, or drugs), the highly reliable occurrence of aggression, the ability to study animals exposed to either aggression or defeat, the clear and valid measures of aggression produced, the simple and sturdy apparatus, and the convenience and economy of data collection.