Glycoprotein: glycosyl transferase activities of rat kidney were studied after subcutaneous injection of cortisone acetate or folic acid, or both. Whereas cortisone acetate had little effect, large increases in glycoprotein: glycosyl transferase activities were found 42 h after administration of folic acid. At this time, sialic acid transferase levels were about twice control values; collagen: glucosyl and collagen: galactosyl transferase activities were increased about threefold and twofold, respectively ; and fetuin: galactosyl transferase activity was almost fourfold higher than control. Some transferase activities were elevated by 17 h after injection of folate and remained above control values 140 h after injection. Cortisone injection had little effect on the folate-induced enzyme changes. These studies are compared with previous results concerning renal hypertrophy after injected folic acid. Although the results are compatible with a nonspecific increased rate of glycoprotein synthesis required for a newly proliferating cell population, the possibility of a more selective action of folic acid on glycoprotein: glycosyl enzyme activities is considered.