1. Isolated peripheral nerves of Carcinus maenas (L.) were capable of maintaining their free amino acid levels steady for the first hour of incubation in saline.

2. The addition of a presumed blood concentration of D-glucose (7.2 mg/100 ml) caused a rapid drop in the amino acid levels, whereas incubation in actual fresh samples of blood did not. It is possible therefore that much of the glucose in crab blood might be effectively sequestered into the haemocyte fraction.

3. It is suggested that metabolism might play an important part in the maintenance of the very steep free amino acid concentration gradients across the neuronal membranes of this species.

4. The efflux of amino acids from this tissue is complex. It consists of a slow component (T0.5 = 77.7 min) and a fast component (T0.5 = 3.1 min) which account for about 50% and 40% respectively of the total intracellular radioactivity accumulated after a 20 min incubation.

5. In experiments where the nerves retained connexion with the thoracic ganglion, the levels of amino acids were higher than in controls which were isolated from the ganglion by a ligature. The possible significance of these findings is discussed.

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