Generally, resistance to extinction is greater after partial reinforcement procedure than after continuous reinforcement. This phenomenon may be explicable mainly by F. D. Sheffield & Temmer's generalization decrement theory (2) and V. F. Sheffield's after-effect theory (3). The former emphasizes the changes of cue pattern from training period to extinction period. due to the elimination of reinforcement. Since these cue changes are smaller in the groups of less reinforcement rate, more persistent resistance is expected in these groups.The latter pays attention to the facts that in partial reinforcement procedures response is also conditioned to the cues involving after-effects of non-reinforcement. Therefore, so far as moderate number of non-reinforcement trials are concerned, the conditioning to such non-reinforcement cues are stronger in the groups of greater reinforcement rate, and greater resistance is expected in these groups. Practically, however, the most favorable rate will be expected in the middle of these two extreme rates.Our experiment was performed to test the above considerations. Rats in Group-1/1 were placed on electrified grill of grey compartment in every trial and allowed to escape from shock by running to the adjacent non-electrified white compartment (escape procedure). Group-1/2 received escape procedure as Group-1/1 in half of the total 40 trials, but the electric shock was eliminated from both compartments in the remaining half trials which were interspersed semi-randomly with shock trials. Group-1/4 received escape procedure in a quarter of trials and was given no shock ni the remaining trials.The results obtained were as follows :1) In the non-shock trials in training process, more speedy runnings were performed by Group-1/2 than by the Group-1/4. However, the difference did not reach a statistically significant level (Table 1).2) The order of running speed in the early extinction stages was : Group-1/2, Group-1/4, Group-1/1. (Fig. 2-c). The differences between Group-1/1 and Partial Groups were significant, and difference between Group-1/2, and Group-1/4 was nearly significant (Table 2).With respect to the number of trials to reach the extinction criterion, the order of three groups was the same as in the case of running speed. The differences between Group-1/1 and Partial Groups were significant or nearly significant, but there was little difference between Group-1/2, and Group-1/4, (Table 2).The following conclusions were deduced from these results :1) The faster extinction in Group-1/1 is attributed mainly to the greater cue change due to elimination of electric shock as a cue. However, one should understand that this cue change is different from change of cue pattern due to elimination of rewards as in reward training situation.2) Though Group-1/2, performed more speedy running than Group-1/4, in early extinction stage, number of trials to reach extinction criterion was almost equal in both groups. This seems to indicate that the less reinforcement rate in Group-1/4, was favorable to persistence of response.