In order to estimate myocardial damage after severe burn injury, the cardiac myosin light chain I (CMLC1) contents and myocardial enzyme spectrum (CK, AST and LDH) in serum were measured in rats with full thickness burn of 30% TBSA. The results showed that the CMLC1 contents in serum increased markedly at 1 h postburn and about 20 times higher than control value at 12 h postburn, remaining at high level at 24 h or 48 h postburn, and decreasing slightly at 72 h postburn. CK, AST and LDH activities were increased at 3 h postburn and tended to decrease markedly at 24 h or 48 h postburn, recovering to control level at 72 h postburn. These results suggest that high increase in CMLC1 in serum implies that myocardium is damaged after severe burn injury, and it can be used as a reliable index in the diagnosis of postburn cardiac injury with higher sensitivity and specificity than myocardial enzyme spectrum.