Kemp, Justin G., Felicia A. Greer, and Larry A. Wolfe. Acid-base regulation after maximal exercise testing in late gestation. J. Appl. Physiol. 83(2): 644-651, 1997.This study employed Stewart's physicochemical approach to quantify the effects of pregnancy and strenuous exercise on the independent determinants of plasma H⁺ concentration ([H⁺]). Subjects were nine physically active pregnant women [mean gestational age = 33 ± 1 (SE) wk] and 14 age-matched nonpregnant controls. Venous blood samples and respiratory data were obtained at rest and during 15 min of recovery from a maximal cycle ergometer test that involved 20 W/min increases in work rate to exhaustion. Mean values for [H⁺], PCO₂, and total protein increased, whereas those for bicarbonate concentration ([HCO₃]) and the strong ion difference ([SID]) decreased in the transition from rest to maximal exercise within both groups. At rest and throughout postexercise recovery, the pregnant group exhibited significantly lower mean values for PCO₂, [HCO₃], and total protein, whereas [SID] was significantly lower at rest and early recovery from exercise. [H⁺] was also lower at all sampling times in the pregnant group, but this effect was significant only at rest. Our results support the hypothesis that reduced PCO₂ and weak acid concentration are important mechanisms to regulate plasma [H⁺] and to maintain a less acidic plasma environment at rest and after exercise in late gestation compared with the nonpregnant state. These effects are established in the resting state and appear to be maintained after maximal exertion.