1. One hundred and two single afferents from the finger extensor muscles of humans were studied with the microneurography technique. 2. The afferents were provisionally classified as primary muscle spindle afferents (62/102), secondary spindle afferents (22), and Golgi tendon organ afferents (18) on the basis of their responses to four tests: 1) ramp-and-hold stretch, 2) 20- and 50-Hz small-amplitude sinusoidal stretch superimposed on ramp-and-hold stretch, 3) maximal isometric twitch contraction, and 4) stretch sensitization. 3. The response profiles of the three unit types were analyzed during slowly rising isometric contraction terminating with an abrupt relaxation. About 75% (61/84) of all muscle spindle afferents increased their discharge during isometric contraction, whereas the discharge was reduced for the remaining afferents. All Golgi tendon organs increased their discharge during the contraction. 4. The level of extrafusal contraction at which a spindle afferent increased its discharge rate often varied from trial to trial, speaking against a fixed fusimotor recruitment level of the individual spindle ending. 5. In 70% of the spindle afferents, a distinct burst of impulses appeared when the subject rapidly relaxed after the isometric contraction. The burst was more common and usually much more prominent with primary than secondary afferents, often reaching instantaneous discharge rates well above 100 Hz. 6. Whereas all Golgi tendon organ afferents displayed an increased discharge during the contraction phase, only one of them exhibited a rate acceleration close to the relaxation phase. However, this response could clearly be identified as being of different nature than the spindle bursts.(ABSTRACT TRUNCATED AT 250 WORDS)