Maintenance of chronic metabolic alkalosis might occur by a reduction in glomerular filtration rate (GFR) without increased bicarbonate reabsorption or, alternatively, by augmentation of bicarbonate reabsorption with a normal GFR. To differentiate these possibilities, free-flow micropuncture was performed in alkalotic Munich-Wistar rats with a glomerular ultrafiltrate total CO2 concentration of 46.5 +/- 0.9 mM (vs. 27.7 +/- 0.9 mM in controls). Alkalotic animals had a markedly reduced single nephron GFR compared with controls (27.4 +/- 1.5 vs. 51.6 +/- 1.6 nl/min) and consequently unchanged filtered load of bicarbonate. Absolute proximal bicarbonate reabsorption in alkalotic animals was similar to controls (981 +/- 49 vs. 1,081 +/- 57 pmol/min), despite a higher luminal bicarbonate concentration, contracted extracellular volume, and potassium depletion. When single nephron GFR during alkalosis was increased toward normal by isohydric volume expansion or in another group by isotonic bicarbonate loading, absolute proximal bicarbonate reabsorption was not substantially augmented and bicarbonaturia developed. To confirm that a fall in GFR occurs during metabolic alkalosis, additional clearance studies were performed. Awake rats were studied before and after induction of metabolic alkalosis associated with varying amounts of potassium and chloride depletion. In all cases, the rise in blood bicarbonate concentration was inversely proportional to a reduction in GFR; filtered bicarbonate load remained normal. In conclusion, a reduction in GFR is proposed as being critical for maintaining chronic metabolic alkalosis in the rat. Constancy of the filtered bicarbonate load allows normal rates of renal bicarbonate reabsorption to maintain the alkalotic state.