A comparative study of the methods of biological assay of vitamin A with rats was carried out in this experiment. The body weights and vitamin A levels in the blood and liver of the deficient animals were measured at different time intervals after they were supplemented with various doses of this vitamin. The increase of body weights of the rats was lineally correlated with the logarithms of dosages of vitamin A supplemented within the range of 1-12 ?g; while the. vitamin A levels in blood or liver was also lineally correlated with it within the range of 1-10 ?g. Moreover, the vitamin A contents in blood and liver themselves were closely interrelated to each other. All of the correlations were statistically significant. The procedure for the biological assay of vitamin A recommended by the Pharmacopoeia of the People's Republic of China was discussed.