1. The effects of oral hydrochloric acid, ammonium chloride, sodium bicarbonate and ammonium bicarbonate on urea and ammonium excretion in rats on a constant diet were studied.

2. Hydrochloric acid acidosis significantly reduced urea excretion in the rat, with an equimolar increase in NH+4 excretion and no change in their sum. In ammonium chloride acidosis, most of the additional nitrogen intake is excreted as NH+4 and a small percentage as urea. The converse holds true after administration of ammonium bicarbonate. The physiological significance of this is discussed.

3. The shift in nitrogen excretion from urea to NH+4 in acidosis is interpreted on the basis of bicarbonate production and utilization. Urea formation utilizes HCO3. For amino acid sources, this utilization is offset by the metabolism of the carbon skeleton, which gives rise to HCO3. When waste nitrogen is excreted as NH+4, no bicarbonate is utilized and the new HCO3, generated by the carbon skeleton, helps to maintain hydrogen ion homeostasis.

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