1. The lateral-line nerves of trout as well as those of catfish are found to discharge impulses spontaneously at a high frequency.
2. The frequency of nerve impulse discharge is measured as a function of the number of participating receptor groups (lateral-line sense organs). A quantitative analysis is made of the contribution to the total response made by each group of sense organs.
3. An analysis of the variability of the response is presented which makes it possible to estimate quantitatively the longitudinal extent of damage to the neuromasts due to surgical manipulation.
4. A method is described for recording the response of a single nerve fiber in the lateral-line trunk.
5. The frequency of the spontaneous discharge from the lateral-line nerve trunk when plotted as a function of temperature according to the Arrhenius equation yields a temperature characteristic of approximately 5000 calories.
6. The variability of the frequency of response as a function of temperature indicates the existence of temperature thresholds for the spontaneous activity of the neuromasts.
7. A possible basis for the spontaneous activity is considered.
It is pointed out that the lateral-line system may serve as a model of the Purkinje cells of the cerebellum.