Rare morphological features of the Leontopithecus cardiovascular system have been reported in the literature. The samples analyzed in this study came from 33 specimens of Leontopithecus from the collection of the Center of Primatology of Rio de Janeiro-FEEMA (CPRJ-FEEMA). Morphometry and stereological data were obtained from all animals. Adult body weights of L. rosalia were the lowest, the greatest being those of L. chrysopygus caissara; body weights of L. chrysomelas and L. c. chrysopygus were similar and in between those of the two former species. Cardiomyocytes (left ventricular myocardium) were bigger in adults than in infants. The myocardium of L. rosalia showed focal fibrosis, fatty vacuoles, and hyalinization. In L. chrysomelas the myocardium showed areas of fibrosis and presence of mononuclear cells. Fibrosis and areas of congestion were observed in L. c. chrysopygus; areas of disorganization and vascular congestion were found in L. c. caissara. In L. rosalia infants, a greater density of vessels per myocardial area and a greater length density of vessels were observed as compared with those of L. chrysomelas. In adults, L. chrysomelas showed greater density of connective tissue in the myocardium than L. c. chrysopygus and L. c. caissara did. In L. rosalia, cardiomyocyte nuclei had a greater area density than those of the other forms of Leontopithecus. These characteristics may explain the faster development of L. rosalia infants as compared with that of L. chrysomelas and L. c. chrysopygus kept under the same handling conditions at the CPRJ-FEEMA.