Survival patterns of NBL/Up mice exposed to 675-800 R of total-body x-radiation depended on direction and rate of exposure. Homogeneous dual exposures administered at 126 R/minute were more effective than those administered at 63 R/minute. Dorsal and dual exposures at 63 R/minute were equally effective, while absorption reduced the effectiveness of ventral exposures. Since cellular repair of hematopoietic tissue was initiated only during exposure, increased homogeneity without increased exposure rate would not increase effectiveness. With rate, direction, and homogeneity influencing the effectiveness of total-body exposures, fallacies inherent in converting total-body exposures to absorbed tissue dose became apparent.