On intravenous injection into rats, 131I-asialo-alpha 1-acid glycoprotein was rapidly removed from the circulation. Five minutes after injection only a few per cent of the injected dose remained in the plasma and 75 per cent was found to be in the liver, where it could be localized in the hepatocytes. Fifteen minutes after injection, however, the 131I reappeared in the plasma after the labeled protein had been metabolized by the liver. One hour after injection less than 5 per cent of the injected dose remained in the liver. Studies with this material in a series of rats using a gammacamera showed the reproducibility of the the time-activity curves of the liver. In contrast, the unmodified radioactive alpha 1-acid glycoprotein disappeared from the blood at a much slower rate. The disappearance curve of the unmodified alpha1-acid glycoprotein could be fitted with the sum of two exponential curves with half-times of 2.88 H. and 27.3 H., respectively. The 131I-alpha1-acid glycoprotein did not accumulate in any tissue except the kidneys and some of the radioactivity was excreted in the urine. The possible application of 131I-asialo-alpha1-acid glycoprotein and the comparison with other radiopharmaceuticals for liver studies in nuclear medicine are discussed.