Krogh's diffusion constant for CO2, KCO2, was determined in respiring muscle tissue at various levels of tissue PCO2, between 10 and 160 torr, using a technique described previously (Kawashiro et atl, 1975). With increasing mean tissue PCO2, KCO2 declined towards an apparently asymptotic value. The relationship between KCO2 (10(-9) mmol-cm(-1)-min(-1)-tor(-1)) and PCO2 (torr) at 37 degrees C could be approximated by the equation KCO2 = 17.3 [1 + 1.72 - exp(--0.027 - PCO2)]-At PCO2 = 0 torr KCO2 exceeded the asymptotic value, which was virtually attained at PCO2 = 100 torr, by more than a factor of two. Thus CO2 diffusion in muscle appears to be facilitated in the low PCO2 range. Specific CO2 production rate of tissue, which was determined simultaneously, did not vary with CO2 in the PCO2 range studied. Effects of facilitated CO2 transport on CO2 exchange in muscle are assessd using simple models. In the presence of CO2 facilitation muscle PCO2 is reduced, particularly during exercise.