Some photochemical and pharmacokinetic properties of riboflavin, lumiflavin and the 2',3',4',5' tetraacetyl, tetrapropionyl, tetrabutiryl and tetrapahnitoyl esters of riboflavin have heen studied. The esters are considerably more photostable than riboflavin but less so than lumiflavin, and appear to he photosensitizers with a hehaviour similar to that of riboflavin, promoting photoreactions of biological targets even in the absence of molecular oxygen. The various flavins display important differences in their pharmacokinetic behaviour. Riboflavin, lumiflavin and the short-chain esters (the acetyl and propionyl esters) are rapidly cleared from serum, and recovered in comparable amounts from the liver and kidneys. These results are in agreement with their hydrophilic or moderately hydrophobic character. In contrast, the longer-chain butiryl and palmitoyl esters exhibit a prolonged retention in serum and undergo a significantly larger accumulation in the liver as compared with the kidneys; they are also found in the spleen. In all cases the tissue uptake of these esters becomes appreciable only after 24 h. These results are coherent with the highly hydrophobic character of these esters, which induce a slow release from serum lipoproteins and have a preferential clearance via the bile-gut pathway, showing affinity for the components of the reticuloendothelial system. These long-chain riboflavin esters will probably have a greater and more persistent risk of photoinduced hepatotoxicity than riboflavin, lumiflavin and the shortchain esters.