The activities of two key enzymes, methionine adenosyltransferase (EC 2.5.1.6) and cystathionine y-lyase (EC 4.4.1.1), involved in the metabolism of methionine to cyst(e)ine have been studied in the liver, heart, kidney medulla, kidney cortex, pancreas, duodenal wall, spleen and skeletal muscle in the neonatallarnb, unweaned lamb, adult sheep, pre-ruminant calf, ruminant steer and adult goat, and for comparative purposes in the adult rat.Methionine adenosyltransferase was widely distributed in the tissues of the ruminant species examined, with specific activities in the order of 10 2 nmol S-adenosylmethionine formed per milligram protein per 30 min. Greatest activities were found in liver, kidney, spleen and duodenal wall in adult ruminants, and also in skeletal muscle in pre-ruminant lambs and calves. Hepatic methionine adenosyltransferase activity declined with age in both sheep and cattle.Cystathionine y-lyase activity was not demonstrable in skeletal muscle or heart in any of the species examined. Greatest activities (in the order of 10 nmol cystathionine degraded per milligram protein per 30 min) were found in liver, kidney and pancreas for all species. Neonatal lamb tissues had the highest activities. In sheep specific activities in liver, pancreas, kidney cortex and spleen declined with age.