An acute experimental arthritis was induced in the right knee joint of adult cats anesthetized with chloralose, and the activity of single fine afferent units of the medial articular nerve was recorded from filaments of the saphenous nerve. All units included in this study were sensitive to local mechanical probing of the medial and anteromedial aspects of the knee joint. The units were identified by their conduction velocity as belonging either to group III (2.5-20 m/s, 48 U) or group IV (less than 2.5 m/s, 29 U). After the identification, the resting activity was measured, and the evoked activity was determined using local mechanical stimulation with glass rods and von Frey hairs, and passive movements of the knee joint. The results were compared with those obtained in a similar study of fine articular units from normal joints. Resting activity was observed in 75% of the group III and 83% of the group IV units. The discharges were irregular and often of high frequency. Both the percentages of units with resting activity and the frequencies of their discharges were more than twice as high as in the control sample. Nearly all units from inflamed joints had low threshold to movements (group III 89%, group IV 72%), and most units responded well to flexion and extension. In contrast, in units from normal joints only 33% of the group III and 10% of the group IV units responded well to passive movements in the normal working range of the joint. In inflamed as well as in normal joints the responses were mostly tonic in character and adapted slowly or not at all when the joint was held in a new position. In normal joints 24% of the group III and 36.5% of the group IV afferent units with local mechanosensitive receptive fields could not be excited by any physiological or noxious joint movements. Such units were only very occasionally seen in the present sample. The average number of receptive fields per unit found in inflamed joints exceeded considerably that in normal units, but no systematic drop in von Frey thresholds was seen when comparing the control sample with the inflamed one. From the above data we conclude that an acute inflammation sensitizes many fine articular units, making them active at rest and increasing the responsiveness more readily to normally innocuous joint movements. Both nociceptors and units with low threshold to movement seem to become sensitized by the inflammation.(ABSTRACT TRUNCATED AT 400 WORDS)