The effects of acute respiratory acidosis and alkalosis on the activated coagulation time (ACT) of both pre-heparinized and post-heparinized blood samples were studied in adult mongrel dogs. The ACT of pre-heparinized samples, compared with the ACT at a normal pH, was significantly shortened with the progression of acidosis. The correlation coefficient between the ACT of pre-heparinized samples and the pH values during acidosis was 0.55 (p<0.01). During alkalosis, however, the ACT of pre-heparinized samples showed no significant changes. On the other hand, the ACT of post-heparinized samples, compared with the ACT at a normal pH, showed significant prolongation with the progression of both acidosis and aklalosis. The correlation coefficient between the ACT of post-heparinized samples and the pH values was -0.74 (p<0.001) during acidosis, and was 0.55 (p<0.01) during alkalosis. These results indicate that, although the mechanism of the alteration of ATC due to the acute respiratory pH changes seemed to be multifactorial, the ACT in the presence of pH abnormality does not reflect the actual blood concentration of heparin correctly.