This study compared the performance of rhesus monkeys on six different manipulation tasks and four food-rewarded object displacement tasks before and after a large, single dose of whole-body X radiation. These experiments were paralleled by investigations of free cage behavior by McDowell (11,12).The demonstration by Davis, Settlage, and ITarlow (5) of differences between the performances of normal and brain-operated rhesus monkeys on a complex mechanical puzzle suggested the use of mechanical puzzles to compare irradiated and nonirradiated monkeys. Many tasks not reinforced by food were selected because of typical clinical findings of posfradiafion anorexia. MKTTfODS SubjectsSixteen rhesus monkeys with nearly identical training histories were used as .S' s in these experiments. After 324 days of training, the animals were randomly assigned to two groups, an experimental group of ten animals, subsequently radiated, arid a control group of six animals. A pparalusLxperimcnls were conducted either in the animal's living cage or in a modified Wisconsin General Test Apparatus (10). The animals were housed in four double-unit living cages constructed of steel bars and covered with corn cribbing. A pair of animals occupied a single cage unit measuring 3 ft. by 3 ft. by 3 ft., and all animals had an equal opportunity to enter outside living cages.