1. This study investigated the muscle-spindle discharge from the pretibial flexor muscles of standing human subjects while they performed maneuvers that altered their reliance on proprioceptive feedback to control balance. Single-unit recordings were made from 100 identified muscle afferents, 81 from muscle-spindle endings and 19 from Golgi tendon organs. 2. With 49 spindle endings the subjects stood on a horizontal platform and with 32, on a platform tilted in dorsiflexion (4 degrees) to ensure that the pretibial muscles were active to maintain balance. When standing freely on a horizontal platform without support or vision, there was little or no electromyographic (EMG) activity in the pretibial muscles, and spindle discharge rates were low (55% active; mean rate for all 49 endings, 4.1 Hz). When standing similarly on the tilted platform, 69% of the spindle afferents were active, and the mean discharge rate was 5.4 Hz. The greater number of actively discharging spindle afferents and the preservation of mean discharge rate despite muscle shortening indicates that the pretibial muscles are subjected to increased fusimotor drive when they are tonically active to maintain balance. 3. The effects of small degrees of body sway induced voluntarily or by an external stimulus were studied with 41 afferents (29 spindles; 12 tendon organs). Activation of the pretibial muscles to compensate for backward sway was accompanied by a spindle discharge that usually exceeded the discharge produced by comparable passive movement. This indicates that the pretibial muscles are subjected to increased fusimotor drive when they are phasically active to maintain balance. 4. To vary the reliance placed on the feedback from proprioceptive inputs, the subjects abruptly opened and shut their eyes, took and released support, or tilted their heads. There were no detectable changes in afferent activity unless the maneuver produced a change in EMG activity in the pretibial muscles and/or body sway. Thirty afferents (26 of 46 spindles; 4 of 7 tendon organs) underwent a change in discharge rate associated with a transient change in posture, as recorded by the force platform, or a change in EMG activity in the receptor-bearing muscle. The discharge pattern of 23 afferents did not show any clear change with these maneuvers. 5. It is concluded that maneuvers that increase the reliance on proprioceptive feedback when subjects are standing quietly do not significantly alter the fusimotor drive to the pretibial muscles in the absence of muscle contraction.(ABSTRACT TRUNCATED AT 400 WORDS)