1. Triceps surae and plantaris (Pl) motoneurons were recorded intracellularly in chloralose or pentobarbital sodium (Nembutal)-anesthetized cats during unfused tetanic contractions of gastrocnemius medialis muscle (GM) produced by stimulating either a cut branch of the GM nerve or the muscle directly. 2. In alpha-motoneurons, during a series of GM twitches at 10/s, contraction-induced inhibitory potentials, probably the result of input from Golgi tendon organs (autogenetic inhibition), rapidly subsided before the end of the series. In contrast, excitatory potentials, probably the result of the activation of spindle primary endings during relaxation from contraction, persisted. 3. In gastrocnemius lateralis-soleus (GL-S) and Pl motoneurons lacking an excitatory connection with Ia afferents from GM, the sustained contraction of this muscle also elicited a declining inhibition. Rapid reduction of contraction-induced autogenetic inhibition was also observed in homonymous gamma-motoneurons. During unfused tetanic contractions lasting 0.5-4s, inhibitory potentials quickly subsided, but an abrupt increase in contractile force elicited a new series of decreasing inhibitory potentials. 4. The assumption that the inhibition induced by GM unfused tetanic contractions was due to activation of homonymous Ib afferents was supported by observations of the effects of electrical stimulation of the GM nerve. In Pl motoneurons lacking an excitatory connection with Ia afferents from GM, repetitive trains applied to the GM nerve, at a strength just above threshold for group I fibers, elicited rapidly declining inhibitory potentials similar to those produced by GM contraction. It was verified that during such stimulation, the amplitude of the group I afferent volleys did not decrease. 5. Reduction of contraction-induced Ib inhibition during sustained GM contraction was still present after a low spinalization of the preparation. As GM tendon organ discharges were verified to persist throughout prolonged contractions, the observed decline of autogenetic inhibition is likely to depend on a spinal mechanism, possibly involving presynaptic inhibition of Ib afferents and/or mutual inhibition of Ib-inhibitory interneurons.