Vitamin A is normally transported in plasma as retinol bound to a specific protein, retinol-binding protein (RBP). Detailed studies were conducted to examine the effects of excess vitamin A on the plasma concentration and metabolism of RBP, and to obtain information about vitamin A transport in the hypervitaminotic state. Two separate experiments were conducted. In the first (Study I, 99 days), plasma RBP and vitamin A levels were compared in three groups of rats fed 0.14 mg (control), 7.3 mg (group 2), or 41 mg (group 3) of vitamin A per day. After day 50 of the study, the administration of excess vitamin A to hypervitaminotic rats (groups 2 and 3) was discontinued and the rats were allowed to recover from vitamin A toxicity. In the second, shorter experiment (Study II), serum vitamin A and RBP levels were compared in control and hypervitaminotic (34 mg of retinyl acetate per day) rats. The rats in this study were also given [3-H]retinyl acetate daily to determine the distribution of retinyl esters and retinol between the lipoprotein and nonlipoprotein protein fractions of plasma. In both studies, administration of large, excessive doses of vitamin A resulted in substantial and significant decreases in the levels of serum RBP. Excessive doses of vitamin A produced fatty liver in the rats, in association with a normal (group 2, Study I) or with a decreased (group 3, Study I) level of RBP in the liver. It is possible that excess vitamin A leads to decreased rates of RBP synthesis in, and of RBP secretion from, the liver. Administration of excessive doses of vitamin A also resulted in elevations of serum vitamin A levels, which were mainly due to large increases in the circulating levels of retinyl esters. In the hypervitaminotic rats, most of the serum vitamin A, and virtually all of the retinyl esters, was found in association with the serum lipoproteins of hydrated density less than 1.21. These results demonstrate that the serum lipoproteins play an important role in the transport of the vitamin A that accumulates in serum in hypervitaminosis A. We suggest that the toxic manifestations of hypervitaminosis A occur when vitamin A circulates in plasma and is presented to membranes in a form other than bound to RBP. Plasma lipoproteins may nonspecificially deliver vitamin A to biological membranes and hence lead to vitamin A toxicity.