Simultaneous measurements of arterial NH4+ concentration, urinary NH4+ excretion, net-acid excretion and the bidirectional release of NH4+ by the kidney have been made in 11 patients with chronic bronchitis in order to determine the influence of changes in arterial gas tensions and/or pH on the renal release of NH4+. The present findings suggest that in this clinical situation both the urinary excretion and the bidirectional release of NH4+ by the kidney is related to the arterial carbon dioxide tension. Although elevated values for arterial NH4+ concentration were found in some patients with respiratory failure this could not be accounted for by an increase in the release of NH4+ into the renal vein. Increasing the concentration of inspired oxygen resulted in a reduction in the urinary excretion of both NH4+ and net-acid and a decrease in the release of NH4+ into the renal vein, despite a marked increase in the extracellular hydrogen ion concentration. The influence of acute changes in renal blood flow on NH4+ release by the kidney is discussed.