1. The activity of single C- and A-fibre cutaneous sensory units was recorded from the great auricular nerve of anaesthetized rabbits to compare the effects of chemical with other forms of stimulation under several experimental conditions. Chemical agents were delivered by close arterial injection. 2. Small intra-arterial injections of bradykinin (0.2 microgram) and a substantial range of capsaicin doses (2-200 micrograms) consistently activated C polymodal nociceptors without exciting other types of C- or A-fibre cutaneous sense organs. 3. Topical application of xylene to the receptive field of polymodal nociceptors evoked a strong excitation which lasted several minutes. 4. The responses of polymodal nociceptors to mechanical, chemical (bradykinin, xylene) and noxious thermal stimuli were suppressed or abolished after large intra-arterial doses of capsaicin. Capsaicin desensitization of polymodal nociceptors to one kind of stimulation often was not paralleled by similar changes in responsiveness to other stimuli. However, on the average, capsaicin desensitization altered responses to thermal, chemical and mechanical stimuli without afferent selectivity. 5. Background discharge developed in C polymodal nociceptors of the rabbit ear following ultra-violet irradiation sufficient to produce evidence of delayed inflammation. Noxious heat and bradykinin injection (0.2 micrograms) evoked more activity from C polymodal nociceptors in the irradiated ears than from control units.
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