Fluid was collected by micropuncture from individual renal tubules of anesthetized rats and its pH determined with the quinhydrone microelectrode. The single glomerular sample and early proximal fluid were isohydric with arterial blood, but later proximal fluid usually showed progressive acidification. The maximum proximal fall in pH was 0.43 u in nondiuretic rats, 0.56 u during profuse glucose or mannitol diuresis, and 0.78 u in rats previously loaded with ammonium chloride and undergoing glucose diuresis. Fluid from the early distal convolution was usually acidified relative to arterial blood but was not significantly different from late proximal fluid. Progressive acidification probably also occurred in the distal convolution. The pH decreased further in the collecting ducts, much more so in the nondiuretic state than during diuresis. The quantitative importance of proximal reabsorption of HCO3– and, by inference, H+ secretion is emphasized. It is suggested that the pH of tubular fluid may increase in the thin descending limb of the loop of Henle, especially in a kidney elaborating a concentrated urine, because of increased concentration of HCO3–.