Alterations in Golgi tendon organs' (Ib afferent receptors) stretch sensitivity were assessed in fatigued gastrocnemius muscle of cats. Muscle fatigue to 60–50% peak tension was induced in 13 cats by electrical stimulation of L7and S1ventral roots. Forty-three group Ib afferent fibers were recorded in L7or S1dorsal root filaments before and during fatigue. Fiber activity was assessed by applying to the Achilles tendon a 5-mm ramp stretch at 5 or 25–30 mm·s-1. Group Ib afferent responses to both velocities of stretch were either completely abolished or depressed over several seconds compared to pre-fatigue firing frequencies. When responding, longer latencies (measured at 5 mm·s-1only) to firing onset occurred during fatigue. Increasing the rate of stretch (250 mm·s-1) to produce a 5-mm stretch approximately equivalent to twitch tension time revealed a depression more selective to static stretch sensitivity. Post-excitation depression of the Ib receptor potential appears to be one possible mechanism. These acute adaptations in Ib afferent discharge to muscle stretch were opposite in direction to those recently reported in fatigued muscle for group la and group II muscle spindle afferents. Functional implications of these results are discussed.