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A. INTRODUCTION AND PROBLEMIn previous studies Harlow, Harlow, and Meyer (4) and Harlow (3) have shown that monkeys learn to solve mechanical puzzles and maintain this performance for many trials without food or any other extrinsic incentive. A manipulation drive was proposed to account for both the learning and the persistence of the behavior. In the present investigation mechanical puzzle performance is measured over a longer period of time than in previous investigations. Brain-operated as well as normal monkeys are used as subjects, and the effect of an interposed food-incentive condition is analyzed. B. SUBJECTSAll had extensive testing experience and, with the exception of the occipital group, identical laboratory histories. Four were normal, four had undergone extirpation of one occipital Jobe, four had been subjected to extensive unilateral decortication (subsequently designated as the EUD Group), and four had undergone extensive unilateral decortication plus removal of the lateral surface of the remaining frontal lobe (subsequently designated as the EUDALF Group).Extensive unilateral decortication involved sparing only of Areas 4 and 6, and 3, 1, 2.2 C. APPARATUST h e essential apparatus was the mechanical puzzle illustrated in Figure 1. It consisted of a board on which were mounted the six devices: a vertical pin, a horizontal pin, a door-stop, a bolt, a hasp, and a hinge. These devices were so arranged that they could be opened only in serial order and, once released, did not automatically close. Actually, closing of the devices by Sixteen macaque monkeys served as subjects. *Received in the Editorial Office on October 18, 1949. 'Supported in part by th,e Research Committee of the Graduate School from funds 9All of the operated monkeys were chronic preparations and had been allowed a supplied by the Wisconsin Alumni Research Foundation. minimum of 90 days postoperative recovery. 305