1. A method is described for the serial determination of renal tubular reabsorption of amino acids in the ethanol-anaesthetized rat. It utilizes intravenous radio-labelled inulins, automated amino acid analysis and forced diuresis.

2. Intravenous loading with phenylalanine and infusion of phenylalanine analogues in this preparation decrease reabsorption of endogenous amino acids in accordance with existing concepts of amino acid transport.

3. Maximal tubular reabsorption (Tmax.) could not be demonstrated for phenylalanine at plasma concentrations below 9 mmol/l.

4. Infusion of phenylalanine analogues into phenylalanine-loaded (‘phenylketonuric’) rats did not specifically inhibit tubular reabsorption of phenylalanine and it is unlikely that any of the substances tested have a potential therapeutic use in man.

5. p-Guanidino derivatives of phenylalanine, in contrast to p-amino derivatives, appear to cause a dose-related basic aminoaciduria.

6. Consideration of urinary flow rates and sodium excretion suggests that the ethanol anaesthesia does not modify amino acid reabsorption through effects on sodium transport or antidiuretic hormone.

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