(1) Habituation of the flexor was studied in unanaesthetized rats.(2) After habituation had been established as a result of the application of 500 uniform stimuli, the time-course of recovery of response was assessed. (3) Recovery was shown to be complete 9 h after the last stimulus had been applied. A linear relationship was shown to exist between the magnitude of the response to the first stimulus applied after the recovery period, and the time allowed for recovery to take place. (4) After partial or complete recovery of response, a second series of 500 stimuli was applied. The rate of habituation to this second series of stimuli was not significantly different to the rate during the initial session, but in those experiments in which the recovery period was 320 min or less, the overall levels of response during the second series were lower than those to the first. (5) Sensitization of the flexor reflex, when tested in spinal rats, was shown to be a relatively transient phenomenon. Responses fell to pre-sensitization levels within 60 sec after stimuli were withheld.