The acid-base status in the toad Bufo viridis was assessed on blood samples taken through a chronical cannula from unrestrained animals. Blood gases and the pH were very constant in animals which were kept either under control conditions (free access to tap water) or immersed in shallow water. pH was 7.646 +/- 0.032 (mean +/- SEM) in 14 control toads at 26 degrees C. PCO2 was 13.1 +/- 1.2 mm Hg and [HCO3-] was 17.0 +/- 0.7 mmol/l at the same temperature. pH and PCO2 were independent of the hematocrit and the haemoglobin in the blood. Diamox induced a characteristic metabolic acidosis which is apprently the result of the inhibition of carbonic anhydrase both in the kidney and in the skin. The results are discussed in relation to the role of the kidney and the skin in regulating the acid-base balance in the toad.