1. Renal bicarbonate reabsorption was measured in thirty new-born dogs 2-27 days of age. Plasma bicarbonate was varied in the puppies by exchanging their blood with blood containing high levels of bicarbonate and low levels of chloride.2. The exchange transfusion resulted in increases of plasma pH, P(CO2) and bicarbonate in the puppies without changing plasma sodium and potassium or glomerular filtration rate (g.f.r.) and body weight.3. There was no tubular reabsorption maximum (T(m)) for bicarbonate and reabsorption values as high as 50 muequiv/ml. g.f.r. could be attained. No animals excreted bicarbonate at plasma levels below 25 mM and some animals had plasma bicarbonate threshold values in excess of 40 mM.4. Bicarbonate reabsorption increased as arterial P(CO2) rose (r = 0.62) but this was due to the rise of filtered bicarbonate since (a) there was no correlation between arterial P(CO2) and bicarbonate reabsorption when factored by filtered bicarbonate and (b) lowering arterial P(CO2) by mechanical hyperventilation did not reduce bicarbonate reabsorption corrected for filtered load.5. Inhibition of renal carbonic anhydrase by acetazolamide (50 mg/kg) resulted in an inhibition of bicarbonate reabsorption of only 4.5 muequiv/ml. g.f.r. (less than 20% of the total). Even with renal carbonic anhydrase inhibited, there was no bicarbonate T(m) and bicarbonate reabsorption values as high as 40 muequiv/ml. g.f.r. could be attained.6. There was good correlation (r = 0.82) between inhibition of sodium and bicarbonate reabsorption during renal carbonic anhydrase inhibition. However, with carbonic anhydrase inhibited, there was no correlation between arterial P(CO2) and bicarbonate reabsorption.7. These results demonstrate that tubular bicarbonate reabsorption mechanisms in the new-born dog are as efficient as those reported for the adult as long as body fluid and plasma sodium and potassium levels are carefully maintained.8. The results are also consistent with a bicarbonate reabsorptive mechanism explained either by direct ionic bicarbonate reabsorption or by hydrogen ion secretion with diffusion of carbonic acid.