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The strength of an instrumental response can be measured by its resistance to change (a) in extinction where reward is withheld or (b) in retraining where a different response is rewarded (7). It has been clearly established that partial reinforcement induces a greater resistance to extinction than continuous reinforcement (4). This generalization, however, has been based solely on studies in which response strength has been assessed by the performance in the absence of reinforcement. The purpose of the present investigation is to test the validity of this generalization when reward is withdrawn from an original response and the organism learns a new instrumental sequence.METHOD Subjects and maintenance.-The S& were 58 experimentally naive male hooded rats of 90-120 days old from the colony maintained by the Psychology Department of the University of California at Los Angeles. Fifteen additional 5s were discarded after varying amounts of training due to excessive timidity, sickness, and experimental error. The Ss subsisted on Little Friskie pellets supplemented by slight amounts of lettuce twice a week.Apparatus.-The cross-shaped alley maze embodied a runway with extended side alleys (see 1 A portion of a dissertation submitted to the graduate division of the University of California at Los Angeles in partial satisfaction of the requirements for the degree of Doctor of Philosophy. The writer would like to express his sincerest thanks to the doctoral chairman, Dr. H. C. Gilhousen, and the committee members, Drs. I. Maltzman, J. P. Seward, F. Fearing, C. Panunzio, and M. V. Seagoe, for their assistance and encouragement throughout the course of the investigation.