1. Ammonia absorption has been studied in closed loops of rat jejunum, ileum and colon.

2. Ammonia absorption was significantly greater from solutions buffered at higher pH. Absorption was also greater in the presence of the bicarbonate ion, and this effect was not due solely to the influence of bicarbonate upon the initial pH of the solutions studied. In the presence of ammonia net bicarbonate absorption was increased and net bicarbonate secretion was decreased.

3. The above effects are compatible with the hypothesis that non-ionic diffusion plays an important role in the absorption of ammonia.

4. Ammonia absorption was significantly less from hypertonic solutions than from hypotonic or isotonic solutions. This effect persisted even after the hypertonic solution had been removed, and therefore appears to have been due to a change in ileal wall permeability rather than an effect of these solutions on water movement. The absorption of hydrazine was similar to that of ammonia in these circumstances; as there is evidence that hydrazine is absorbed by a process of non-ionic diffusion in the lipid phase, this finding is a further indication that hypertonicity exerts its effect through a change in membrane permeability.

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