The effect of digitoxin and ouabain in vitro on the ATP-ase activity of cardiac muscle was studied. The ATP-ase activity was measured by the method of DuBois and Potter (1943). Aqueous solns. of ouabain were added to give a final cone, of 6 X 10-5 [image], and 10% ale. solns. of digitoxin were added to give a final conc. of 4.7 X 10-6 [image]. The reaction rate was limited by the Ca conc. with Ca below 0.003 [image]. A final conc. of 4.7 X 10-6 [image] digitoxin inhibited the ATP-ase activity of cardiac muscle. The % inhibition decreased with increasing Ca cones. In the presence of 0.003 [image] Ca, 6 X 10-5 [image] ouabain produced 13.8% inhibition. The amt. of inhibition was independent of the Ca conc.