Studies of radiation effects on maze learning in rats indicated that increased hunger drive following radiation sickness can account for the facilitation of maze performance in brain irradiated rats. Detrimental effects of brain irradiation can also be masked by increased hunger drive following radiation sickness. Femur irradiation did not produce effects comparable to those of brain irradiation either in terms of radiation sickness and consequent weight loss or in terms of maze performance. (P.C.H.)