To construct acid-base curves and alignment nomograms for swine blood, we added known amounts of acid or base to aliquots of swine blood with three different concentrations of hemoglobin. Pairs of blood samples were equilibrated at two different CO2 partial pressures, and blood pH was measured. Data were analyzed by computer, and mean values were created for complete data of four swine. Curve and alignment nomograms were constructed by computer. The resultant nomograms for swine blood differ from those constructed by Siggaard-Andersen for human blood, most importantly at base-excess values of +10 to +25 mmol/l. The possible reasons for the observed differences are discussed and, although not completely resolved, may be related in large measure to the accuracy of pH determination and the methodology of nomogram construction.