The discharge of 38 tibialis anterior (TA) muscle spindle endings was recorded at rest and during isometric voluntary contraction of the ankle joint in eight healthy human subjects. With the ankle joint in 110° plantarflexion, 24 endings (61%) were tonically active in the resting subject. During weak voluntary dorsiflexion, seven additional endings were activated, so that a total of 31 endings were active (82%). 24 of these were either newly recruited or discharged at a faster rate than at rest (average discharge rate 6.6 Hz at rest, 9.7 Hz during contraction). At matched levels of TA EMG, one ending was newly recruited and 10 were more active during co-contraction of dorsi and plantar flexors than during isolated dorsiflexion. 26 endings were equally active during the two tasks and one ending decreased its firing rate with co-contraction. Four of the 11 endings, which had a higher discharge rate during co-contraction than during dorsiflexion, discharged faster during plantarflexion than at rest although slower than during co-contraction. Plantarflexion had no effect on the discharge of three endings. The remaining four endings were not investigated during plantarflexion. It is suggested that the increased discharge rate of muscle endings during co-contraction is caused either by small changes in the length of the TA muscle or by a disproportionately high fusimotor drive during co-contraction.