The concentrations of S-adenosylmethionine (AdoMet), S-adenosylhomocysteine (AdoHcy), and various methyltransferases were determined in the cerebrum, cerebellum, and liver of rats during development and aging. The liver contained from 3 to 7 and from 10 to 15 nmol AdoHcy per gram in young and adult rats, respectively. The AdoMet concentration was 60 to 90 nmol/g liver from rats of the same age and sex. It did not vary significantly with age. In the brain the AdoMet concentration was 45 to 50 nmol/g at birth and decreased to 20 nmol/g tissue with maturity of the organ. The level of AdoHcy in this organ was less than 1 nmol/g tissue throughout the life-span of the rat. Since the ratio of AdoMet to AdoHcy is relatively high, the rate of methylation of histones, DNA, or phosphatidylethanolamine in the liver or brain was not significantly influenced by AdoHcy. Under normal nutritional conditions, the tissue concentration of AdoMet is far above the K_m values of histone and phosphatidylethanolamine methyltransferases. The levels of activity of these enzymes in liver and brain did not correlate with the cellular concentration of AdoHcy. The histone methyltransferase activity was elevated in rapidly proliferating tissues and declined markedly in the absence of histone biosynthesis. Phosphatidylethanolamine methyltransferase activity was elevated during development of the liver. The specific activity of the AdoHcy hydrolase remained relatively constant in the rat brain and liver. The activity of this enzyme was 10 times higher in liver than in brain, yet the concentration of AdoHcy was much lower in the latter organ. The tissue levels of this compound are evidently dependent on the rates of removal of homocysteine and adenosine. Adenosine deaminase was present in the liver and brain at relatively high concentrations, particularly during development.