[en] A double isotope method for the simultaneous determination of S-adenosyl-methionine (SAM) and catechol O-methyltransferase (COMT) activity in red blood cells was developed. Healthy persons contained 7.9 +- 1.9 nmol SAM per ml red blood cells and 1.9 to 3.8 nmol per ml plasma. Epinephrine increases the SAM biosynthesis rate in red blood cells. The possible influence of endogenous SAM on previous methods to determine COMT activity in red blood cells is discussed. The statement of Briggs and Briggs (Briggs, M.H. and Briggs, M. (1973) Experientia 29, 278-281) that COMT activity in red blood cells is lower during the last 3 months of pregnancy could not be verified. (Auth.)