Diatrizoate concentrations in the blood and twelve tissues and organs of 20 rats were measured at 40 seconds, and 2, 5, 15 and 60 minutes after bolus intravenous administration of 125I-labeled diatrizoate and 131I-labeled albumin. Contrast enhancement (HU) and distribution volumes (%) were calculated from this data, and the results are presented graphically. The tissues can be separated into two groups according to the rapidity of extravascular contrast medium uptake. The liver appeared to concentrate the contrast medium. Curves of local contrast enhancement with time could be constructed by a CT scanner from the data of several consecutive scans and are of potential importance in the differential diagnosis of lesions detected on CT. Repeat postcontrast CT scans could be used to detect and characterize lesions which raise a diagnostic problem. The potential diagnostic advantages of calculating percent distribution volumes from CT scans are demonstrated and discussed.