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1. An electrophysiological study was made with the sucrose gap of the compound action potential of the non-myelinated fibres of the garfish olfactory nerve at temperatures between 8 and 22 degrees C. The efflux of 42K with stimulation, and the post-tetanic hyperpolarization following a period of repetitive stimulation, were also studied. 2. At 22 degrees C the compound action potential (at zero conduction distance) reached its peak in about 5 msec and lasted about 12 msec; the conduction velocity was 19.4 cm/sec. The corresponding extrapolated values at 0 degrees C were: time to peak, 16 msec; duration, 37 msec; conduction velocity, 4.2 cm/sec. 3. The rate constant for resting potassium efflux (kr) was 0.0108 min-1 at 22 degrees C and 0.00489 min-1 at 0 degrees C. 4. The extra fractional loss of potassium with stimulation (ks) was 2.02 x 10(-4) impulse-1 at 22 degrees C and 4.41 x 10(-4) impulse-1 at 0 degrees C. 5. Raising the external potassium concentration, or addition of ouabain (1 mM), raised the resting rate of efflux of potassium. 6. The post-tetanic hyperpolarization, which reflects the electrogenic extrusion of sodium ions, indicated a rate constant for active sodium efflux at 22 degrees C of 0.47 min=1. 7. The rate constant was decreased by a 10 degrees C fall in temperature by a factor of 1.73. 8. Increasing the external potassium concentration increased the rate constant, apparently by potassium combining with some site with an equilibrium dissociation constant of 1.2 mMM. 9. The rate constant was decreased slightly in acidic solutions and increased slightly in alkaline solutions. 10. 14C-labelled ouabain was found to bind to the garfish olfactory nerve with an equilibrium dissociation constant of about 0.5 muM. The maximum saturable binding capacity, 22.3 p-mole/mg dry, suggests that there are about 350 sodium pumping sites per square micron membrane.