Forty-eight rats were trained in the runway for 10 trials each day for 18 days under a 50% partial reinforcement schedule, confinement duration on the non-reinforced trials being either 10 sec. or 50 sec. Half of each group was extinguished under 10-sec. confinement (10-10, 50-10), half under 50-sec. (50-50, 10-50), there being seven days of extinction at six trials each day. Terminal extinction level was affected by confinement. A variety of hypotheses were applied to the results.