1. The effect of metabolic acidosis simulated in vitro on ammoniagenesis was investigated by using the isolated kidney of the rat perfused with an albumin Krebs—Henseleit medium containing glutamine and glucose.

2. Addition of HCl to a perfusate of normal bicarbonate concentration resulted in a prompt increase in urine flow rate, decrease in fractional sodium reabsorption and decrease in urine pH.

3. A minimum urine pH as low as 5·15 was achieved, with an average value of 5·92, indicating that this preparation has the capacity to acidify normally.

4. In contrast with studies in vitro with other preparations, with the functional perfused kidney a diminution in perfusate bicarbonate concentration resulted in a prompt increase in ammonia production, which was strikingly correlated with the decrease in urine pH.

5. The increase in ammonia production was diminished in studies carried out with a non-urinating kidney, in comparison with those that exhibited significant urine acidification.

6. These data suggest that a decrease in urine pH with trapping of ammonia in the urine may be a critical stimulus for increased ammonia production in acute metabolic acidosis.

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