Observations on temporal variation in monosynaptic reflex response in the acutely decapitate cat indicate the following:
1. Frequency distribution of response amplitude has a nearly normal form often with some degree of negative skewness. Response variation differs only moderately in form and magnitude from one preparation to another.
2. Temporal variation remains essentially constant at different levels of drive above that level required to complete the zone of variation.
3. The role of response variation in the determination of mean response amplitude is considered.
4. One of the major sources of excitability fluctuation in the "resting" cord is variation in background activity of interneurons.