The paired shock technique was used to study the effect of sleep on the excitability of the short latency R1 and long latency R2 components of the electrically elicited blink reflex. During wakefulness, R1 returned, after transient potentiation, to its original level in about 150 ms following the conditioning stimulus. Contrastingly, R2 remained profoundly suppressed up to 800 ms, suggesting sustained reduction of excitability of the polysynpatic reflex pathways after the passage of a preceeding impulse. During non-REM and REM sleep, the recovery curve of R1 was similar in character, although different in time course to the one obtained during wakefulness. However, R2 was potentiated rather than suppressed by the conditioning stimulus during both phases of sleep. These findings indicate that, during sleep, the polysynpatic reflex pathways are not inhibited by a preceeding impulse to the same degree as in wakefulness.