In situ perfused hearts of carp were used to investigate the roles of the pectoral musculoskeletal framework covering the pericardial cavity, and the pericardium in cardiac performance. Removing the pectoral musculoskeletal framework decreased stroke volume and cardiac output to about 70% and 67% of the initial condition, respectively, and raised atrial filling pressure from -2.9 +/- 0.48 mmH(2)O to above ambient levels. Cutting the pericardium, however, caused no further significant changes in cardiac parameters. Thus, the pectoral musculoskeletal framework of carp is a significant component of the atrial filling via the vis-a-fronte mechanism. On the other hand, the ability of the heart to undertake pressure-related work was significantly enhanced by both removing the pectoral musculoskeletal framework and cutting the pericardium. These results show that both the pectoral musculoskeletal framework and the pericardium prevent the cardiac chambers from over-distention and excessive pressure-related work.