The stroke output of the heart was determined by measurement of the amount of blood in the ventricle at systole and diastole in four species of fish, the bowfin, Amia calva L., the common sucker, Catostomus commersonii (Lacépède), the carp, Cyprinus carpio L., and the catfish, Ameiurus nebulosus (Le Sueur).The stroke output varies with the size of the individual. Smaller individuals of a species, although possessing a smaller absolute output, have a larger output in relation to their size than do larger individuals.Over the size range at which they can be compared (300 to 600 gm.) the four species differ in their stroke output. The catfish has the highest output and the sucker the lowest. The bowfin and the carp are intermediate in this respect. At 500 gm., the outputs of the catfish, bowfin, carp, and sucker per stroke are respectively 0.26 gm., 0.22 gm., 0.18 gm., and 0.11 gm.The results of the present investigation were correlated with those of Black (Biol. Bull. 79: 215–229, 1940); the output of the heart was found to correlate inversely with the effect of carbon dioxide on the blood of the same species, and directly with the affinity of the blood for oxygen. Differences in the circulation may compensate for differences in oxygen transport imposed by the varying effects of carbon dioxide on the blood.