In the 11th in a series of reports on the effect of convulsive treatments on conditioned fear in rats, 84 male, albino rats received a total of 8 conditioning trials which established a strong conditioned fear response. Then the animals were divided into 7 groups (12 rats each) and given a total of 21 electro-convulsive shock treatments (ECS) as follows: Grp. I, 1 per sec.; grp. II, 1 each 30 min.; grp. III, 1 each hr.; grp. IV, 1 each 8 hrs.; grp. V, 1 each 24 hrs.; grp. VI, 1 each 48 hrs.; and grp. vn, 1 each 72 hrs. Retention of the conditioned fear response was tested two days after the last treatment for each group. ECS had no discernible effect on the conditioned response in grp. I and only weakened the response somewhat in grps. II, VI, and VII. But the conditioned response was virtually absent after treatment in grps. III, IV, and V, indicating that the attenuating effect of ECS on the conditioned fear response is at least in part a function of the temporal distribution of the treatments.