The movement of 131 I-labeled homologous albumin from the vascular to the peritoneal compartment of rats during the hours following whole-body X-irradiation (750 R or 1500 R) is described. The concentration of labeled protein in peritoneal fluid after a short circulation time (30 minutes) increased rapidly following irradiation; this phenomenon did not reflect decreased peritoneal fluid volume or decreased peritoneal clearance of the labeled protein. The increased in relative peritoneal radioactivity was clearly biphasic after 750 R, with maxima of 62% and 79% above controls 4 to 8.5 hours and 48 hours after irradiation, respectively. After 1500 R, the effect was more sustained and reached a much higher level (230% above controls at 48 hours). The initial portion of the response was not dose-dependent, although the later phase was clearly so.