Increase in efflux of inorganic phosphate during electrical activity in small non‐myelinated nerve fibres.

1. The movements of labelled phosphate were measured in garfish olfactory and in rabbit vagus nerves at rest and during activity. 2. In garfish olfactory nerve kept in solutions with 120 mM‐sodium and 0.2 mM‐phosphate the fractional loss of 32P was 9.82 X 10(‐4) min‐1. Lowering the sodium concentration of the washing fluid decreased the efflux; lowering the phosphate produced a transient increase with subsequent return towards the efflux in 0.2 mM‐phosphate. 3. Stimulation at 0.50 sec produced an extra fractional loss of 12 X 10(‐6) impulse‐1. At 1/sec the effect was larger; at 5/sec it was about the same as at 0.5/sec. 4. After stimulation the effect of activity disappeared exponentially with a time constant of 4.4 min. 5. Lowering the sodium decreased the extra efflux with stimulation, whereas changing the phosphate concentration did not much affect the extra efflux. 6. In rabbit vagus nerve kept in 154 mM‐sodium and 0.2 M‐phosphate the fractional loss of 32P was 4.91 X 10(‐4) min‐1. Lowering the sodium or the phosphate decreased the resting efflux. 7. Stimulation of the vagus nerve at 15/sec produced an extra fractional loss of 0.87 X 10(‐6) impulse‐1. 8. The extra efflux with stimulation seems to result predominantly from an increase in intracellular inorganic phosphate resulting from increased break‐down of ATP after activity.