when the animal failed to avoid, was 55-V ac through lOK resistance. Goalbox detention comprised 30 sec of the 60-sec intertrial interval. The Ss were 55 female hooded rats, 84-105 days old, trained and extinguished in a single session. The extinction criterion was 60 sec without entering the goalbox.ROBERTS.BEECROFT University of North Dakota, Grand Forks, N. Dak. 58201Avoidance extinction was analyzed by classifying running times as fast, medium, or slow and determining the contingent probabilities of runs of various speed or extinction on the following trial. The most probable transitions were to runs of the same speed class. Extinction was much more probable following slow runs than after medium or fast runs. Increasing the amount of avoidance training contributed to resistance to extinction by lengthening the sequence of fast runs at the beginning of extinction.