Sarcoplasmic reticulum vesicles isolated from were rabbit hindleg and back muscle, yielding 120-130 mg sr prote in using 9 resulting in muscle, 200-300 and a (ca2++mg2+) -atpase with specific activity of 13-15 .umol/mg/min. ncd-4 and efficient as shown to be ncd-5 were fluorescent probes of(ca2++mg2+)-atpase. both carbodiimides inhibited activity of in the only the absence enzyme ca2+(specifically labelled vesicles) :inclusion of ca2+(1 mm) protects against inhibition. a series of phenolic compounds showed an inhibitory action on(ca2++mg2+)-atpase activity in the following order: bis(2-hydroxy-3,5-methylphenyl)methane> 3(4'-hydroxyphenyl) de cane and (4'-hydroxyphenyl-n-heptylketone> bis(4 1,1 hydroxyphenyl)cyclohexane> 4, 4 '-isopropylidene diphenol> bis(2-hydroxyphenyl)methane> 4,4'droxybenzophenone>4 methoxyphenyl-n-heptylketone. correlation was good no obtained between the ability of phenols to inhibit(ca2++mg2+) atpase activity and the ability to perturb a ca2+-induced fluorescence quench leading to the suggestion that phenolic compounds affect other steps in the catalytic cycle besides those associated with ca2+-binding.cyclopiazonic acid found be to an excellent was inhibitor whereas its analogs "a" and" b" showed no effect on(ca2++mg2+)-atpase activity: this may possibly ascribed tothe high atp concentration used in the assay. all flavanoids examined exhibited an inhibitory effect on the(ca2++mg2+)-atpase activity in the following order: robinetin quercetin luteolin fisetin galangin rhamentin. this group of compounds required for their potency a 7 hydroxyl group but in the presence of additional hydroxyl groups at 3'4' and 5'positions. hydroxyl groups 3 and 5 seem do participate inhibitory the in the potency of not flavonoids. quercetin was inhibit(ca2++mg2+)-atpase activity found to reversibly with half- aximal this inhibition at 10 p.m. inhibition showed a ph dependence with the doses of quercetin being significantly lower at ph 8 than at phs 7 and 6.4. also this inhibition can be removed by affinity chromatography on a reactive red-120 agarose matrix. irradiation with light of wavelengths> 350 nm of a mixture of (ca2++mg2+) -atpase and quercetin led to covalent inhibition of enzyme, whose extent is related to quercetin and enzyme concentrations. pre-irradiation followed by of uercetin incubation at room temperature for 30 min with(ca2++mg2+) atpase did inhibit enzyme. the possibility on not photolabelling of the enzyme is discussed.