The relationship between conditions of exposure to carbon monoxide (CO) and biochemical effects was investigated in experiments on rats. The magnitude and the time of biochemical disturbances in the tissues resulting from two different exposures consisting of 1 Vol.-% CO for 4 min and 0.4 Vol.-% CO for 40 min respectively were compared. In both cases, at the end of exposure the same level of blood carboxyhemoglobin (COHb) (about 50%) was reached. The biochemical determinations in the blood (pH, glucose, lactate, pyruvate) and brain tissue (lactate, pyruvate) were carried out immediately after termination of the exposure and after the time periods of restitution. CO exposure resulted in a decreased blood pH, increased level of blood glucose, as well as that of lactate and pyruvate both in blood and brain tissue. These changes were much more pronounced following the "longer-lesser" exposure than after the "shorter-intense" one, although blood concentrations of COHb was the same. The observed phenomenon puts some light on the frequently encountered lack of the correlation between COHb level in blood and severity of CO intoxication in clinical practice.