This investigation provides direct evidence that rat pineal gland contains ATP:L‐methionine S‐adenosyltransferase. The specific activity of the enzyme is about 25 times greater than that of hydroxyindole‐O‐methyltransferase, the unique marker enzyme in this tissue. The major product of the reaction catalyzed by ATP:L‐methionine S‐adenosyltransferase has been identified as S‐adenosyl‐methionine which can function as a methyl group donor for the formation of melatonin. The specific activity of pineal ATP:L‐methionine S‐adenosyltransferase is at least eight times greater than that of brain, and one‐half that of the liver enzyme.