A high-performance liquid chromatographic method, with ultraviolet detection, for the determination of S-adenosyl-L-methionine and S-adenosyl-L-homocysteine, and simultaneously of normetanephrine and metanephrine, is presented. The separation was carried out by reversed-phase ion-pair isocratic chromatography. This procedure was applied to the study of the content of S-adenosyl-L-methionine and S-adenosyl-L-homocysteine in rat brain and adrenal glands and to the measurement of phenylethanolamine-N-methyltransferase or catechol-O-methyltransferase activities in rat adrenal gland.