Late responses were studied in the flexor carpi radialis muscle by surfacerecording and single-fiber electromyography. By single-fiber studies we were able to distinguish 2 distinct components. One was an H reflex obtained without a preceding M response and with a stimulus response jitter of about 100 Lsec; its latency was shortened and jitter decreased with the Jendrassik maneuver. The other was an F wave always preceded by an M response and with a stimulus response jitter of under 50 psec; its jitter and latency are unaffected by the Jendrassik maneuver. At the single-fiber level it was possible to determine the rate of occurrence of individual H reflexes and F responses for a given number of surface-applied stimuli. The H reflex had a very high rate of occurrence (up to 97%), whereas the F wave occurred very infrequently (less than 2% of the time). Using the H reflex rate of occurrence in conjunction with the maximal surface HIM amplitude ratios allowed us to determine the proportion of alpha motor neurons which participate in the H-reflex generation. The study of H-reflex jitter gives an estimate of the central synaptic jitter and effectiveness of the spatial and temporal summation of type 1 a fiber inputs on the motor neurons.