The arterial pH decreases with rising body temperature in ectothermic vertebrates. We report on how this regulation was achieved in relation to bimodal respiration in the toad Bufo paracnemis. Gas exchange was measured for the lung and also for the whole body (skin and lung). In addition, lung gas pressures (PL(O2) and PL(CO2)) and arterial blood gases (pH and P(O2)) were measured at 17, 27 and 35 degrees C. Arterial pH fell from 7.85 at 17-7.64 at 35 degrees C. Concomitantly, PL(CO2) increased from 5.6 mmHg at 17 degrees C to 10.4 mmHg at 35 degrees C. Regardless of temperature, PL(O2) remained virtually constant at 125-126 mmHg, whereas arterial P(O2) increased significantly with rising temperature (50 mmHg at 17 degrees C; 79 mmHg at 35 degrees C). The pulmonary gas exchange ratio (RE) increased from 0.28 at 17 degrees C to 0.51 at 35 degrees C, while the ratio for whole body gas exchange (lung + skin) was close to 0.8 irrespective of temperature. Since CO2 conductance of the skin increased little with temperature, the lung eliminated a larger fraction of total CO2 output. This shift caused the increase of RE for pulmonary exchange with rising temperature, which increased PL(CO2) and contributed to a negative deltapHa/deltaT. Therefore, bimodal respiration in Bufo accounted for part of the temperature-dependent pH regulation, while the final adjustments depend on pulmonary ventilation.