Male Wistar rats were fed either a balanced diet whose protein source was a mixture of amino acids (diet 1), or a similar diet which differed only in having 48% less the quantity of the sulfur-containing amino acids methionine and cystine (diet 2). The diets were given either continuously for 1 month or for 15 days after a protein-free diet. Both diets 1 and 2 permitted good growth of rats and relatively stable microsomal protein content. Protein depletion decreased the total proteins, total phospholipids, and cytochrome P-450 content, and it strongly increased UDP-glucuronosyltransferase activity in rat-liver microsomes. Repletion with diet 1 restored those values, to the level found in control rats. However, diet 2, given continuously induced an increase in UDP-glucuronosyltransferase activity and in the cytochrome P-450 concentration. Since high UDP-glucuronosyltransferase activity was related to lessened amounts of sulfur-containing amino acids in the diet, we discuss the possible effect of methionine and cystine on the regulation of glucuronoconjugation in relation to sulfoconjugation.