The paper investigates the effect of two potential beta-adrenolytical agents, 1-[4-(3-isopropylami- no-2-hydroxypropoxy)-3-(propoxymethyl)phenyl]etanon (FA33) and 1-[4-(3-isopropylamino-2-hyd - roxypropoxy)-3-(propoxymethyl)phenyl]propanan-1-on (FP33), in the form of salts with fumaric acid on the enzymatic activities of the markers of subcellular fractions of the rabbit cardiac muscle (in vitro). In the mitochondria, both substances inhibit the specific activity of cytochromoxidase (FA33 by 37.14 % and FP33 by 35.88 %). In the sarcolemma, substance FA33 inhibits the specific activity of Na + -K + -ATPase by 23.02 %, while substance FP33 possesses an activating effect increased by 23.24 %. In the sarcoplasmatic reticule both substances inhibit the specific activity of Ca 2+ -Mg 2+ - ATPase (FA33 by 14.96 % and FP33 by 8.79 %). The different effects of substances FA33 and FP33 on the activities of the markers of the subcellular fractions of the cardiac muscle can be connected with different inotropism of both substances.

        Key words: beta-adrenolytic agents – subcellular fractions – cytochromoxidase