The values obtained from the measurement of serum creatine phosphokinase (SCPK) in 70 cases of chronic cor pulmonale in exacerbation and 72 cases in remission were compared with the normal value as measured in 160 healthy adults at the same period. The results showed that the normal value of SCPK was 46.50 +/- 12.71 u/L in male, while 32.51 +/- 10.8 u/L in female. The differences of the values of SCPK in male and female are highly significant statistically (P less than 0.001). Although the values of SCPK were within normal limits (M +/- 2SD) in most patients with chronic cor pulmonale (72.38-84.10%) in this paper, the values of SCPK of some of them (13.60-22.36%) were higher than normal. But in a few of the patients in exacerbation (2.27-5.26%) the SCPK values were lower than normal. The increased activity of SCPK in patients with chronic cor pulmonale may be related to the vigorous movement of the respiratory muscles. While the level of SCPK decreased to extremely low degree may indicate that the respiratory muscles may be excessively fatigue. So we suggest that the levels of SCPK which are markedly higher or lower than normal could be served as an reference index for the excessive activity or fatigue of the respiratory muscles respectively in chronic cor pulmonale.