This study was initiated to obtain data on the rate of carbon monoxide (CO) uptake (theta CO) by hemoglobin in pig erythrocytes to derive, in a later study, the pulmonary capillary blood volume (Qc) in pigs from the Roughton-Forster relationship. Blood from five different female pigs was used. The theta CO, the milliliters of CO taken up by 1 ml of whole blood per minute per Torr CO tension, was determined on each blood sample with a continuous-flow rapid-mixing apparatus and double-beam spectrophotometry at 37 degrees C and pH 7.4 at four or five different PO2 values. Because the individual regression lines of theta CO vs. PO2 were not significantly different, a common regression equation was calculated: 1/theta CO = 0.0084 PO2 + 0.63. The slope of this regression line is significantly steeper than the reported slopes of regression lines for human and dog erythrocytes measured under the same conditions. Our results revealed that calculation of Qc in pigs by using theta CO values for human or dog erythrocytes would result in an underestimation of 51 and 50%, respectively.