1. A study has been made of the temperature changes associated with the passage of a single impulse in the non-myelinated fibres of the pike olfactory nerve. 2. The initial heat occurs in two phases: a burst of positive heat, followed by an evolution of negative heat. The positive and negative heats, and the net initial heat, are temperature-dependent. 3. At 0 degrees C the measured positive initial heat is 44.2 mucal/g.impulse; and the corresponding negative initial heat is 48.9 mucal/g.impulse. There is thus a net initial heat that is negative, of about 4.7 mucal/g.impulse. 4. The positive heat has a positive temperature coefficient, being increased by a factor of 1.86 when the temperature is rasied from 0 degrees C to 10 degrees C. 5. The negative initial heat also increases when the temperature is raised, but less than the positive initial heat. As a result, the net initial heat tends to become positive at higher temperatures. 6. Because of temporal dispersion of the action potential over the face of the thermopile, the observed temperature changes are smaller than those that occur at a single point in the nerve close to the stimulating cathode. The value of the positive heat at 0 degrees C corrected for temporal dispersion is estimated to be about 62 mucal/g.impulse: the corresponding value for the negative heat is about 67 mucal/g.impulse. 7. All records were analysed in terms of only two phases of initial heat (one positive, one negative). No analysis required four phases; but it is unclear whether this finding reflects a true absence of four phases, or merely the inability of the recording equipment to resolve them. 8. The positive heat seems to be derived from two sources. First, there is a dissipation of the free energy stored in the membrane capacity. Secondly, there is an evolution of heat corresponding with a decrease in entropy of the membrane dielectric with depolarization.