To investigate the effects of eccentric exercise on the signalling properties of muscle spindles, experiments were done using the medial gastrocnemius muscle of cats anaesthetised with 40 mg/kg sodium pentobarbitone, i.p. Responses were recorded from single afferent nerve fibres in filaments of dorsal root during slow stretch of the passive muscle and during intrafusal contractions at a range of lengths, before and after a series of eccentric contractions. The sensitivity to slow stretch was measured as the average firing rate between muscle lengths 10.5 and 9.5 mm shorter than the physiological maximum (Lm), during stretch at 1 mm/s over the whole physiological range. The mean sensitivity of both primary and secondary spindle endings increased slightly, but not significantly, after a series of 20-150 eccentric contractions consisting of a 6 mm stretch, at 50 mm/s, to a final length of between Lm -7 mm and Lm, during stimulation of the whole muscle or sometimes of single fusimotor fibres. Discharges were recorded from primary endings during fusimotor stimulation at 100-150 pulses/s, and from secondary endings during static bag intrafusal contractures produced by i.v. injection of 0.2 mg/kg succinyl choline. Spindle responses were recorded, over a range of muscle lengths, in steps covering the whole physiological range. About half of the responses showed a peak in the relation between length and net increase in firing rate, while the remainder either progressively increased or progressively decreased over the physiological range. No large or consistent changes were seen after the eccentric contractions. It is concluded that the intrafusal fibres of muscle spindles are not prone to damage of the kind seen in extrafusal fibres after a series of eccentric contractions.