Shielding of the abdomen during x-irradiation was used to test the hypothesis that the initial decrease in the volitional activity of rats observed after whole-body x-irradiation is related primarily to irradiation of the abdomen. Exposure of the abdomen to 200 r, or of the remaining half of the body to 200 or 600 r did not result in a significant depression in activity. When the abdomen was exposed to 600 or 750 r, however, a depression in activity was observed, with a minimum on the 3rd day postirradiation, followed by increasing activity. When animals were exposed to relatively high doses (1500 or 2000 r) with the abdomen shielded, or with only the head exposed (1500 r), there was an immediate depression in activity on the 1st day postirradiation, then a transient increase in activity to values equal to or well above control levels, followed by a rapid decrease below control activity, and then by increasing activity. The transient increase above control activity was duplicated by pair-feeding nonirradiated animals to the head-exposed group. It appears that the initial decrease in volitional activity may be mediated by altered gastrointestinal function, particularly with respect to the 3rd day postirradiation. In abdomen-shielded or head-exposed animals subjected to higher doses of x-rays, the transient rise in activity appears to be related to restricted food intake, probably as a result of radiation damage of the oral mucosa. The subsequent depression in volitional activity in these animals may be related to central nervous system injury.