In chloralose-anaesthetized cats, the impulse activity of single afferent units conducting at less than 30 m s-1 and having receptive fields in the triceps surae muscle or the calcaneal tendon, was recorded from thin filaments of the dorsal roots L7 and S1. The receptive fields of the units were tested with a variety of graded natural stimuli (local pressure, stretch, contractions, temperature changes). In addition, the algesic agent bradykinin was injected into the receptive fields, but the sensitivity of the receptors to this substance was not used for classification purposes. Four types of receptors could be distinguished using the strongest response to innocuous natural stimulation as the criterion for characterizing a given ending: (a) nociceptors showing no response to innocuous forms of stimulation and requiring noxious (tissue-threatening) stimuli to be clearly activated; (b) low-threshold pressure-sensitive receptors responding to innocuous indentation of the tissue but being relatively insensitive to stretch and contractions; (c) contraction-sensitive receptors reaching high discharge frequencies during active contractions of moderate force and innocuous stretch, but being relatively insensitive to local pressure stimulation; (d) thermosensitive receptors responding strongly to small changes in temperature without reacting to innocuous mechanical stimulation. The possible involvement of the different receptor types in central nervous functions (nociception, mechanoreception, ergoreception, thermoregulation) is discussed.