1. Skin blood flow has been imaged during stimulation of fine nerve filaments containing small numbers of identified C fibre units. Filaments were dissected from the saphenous nerve of anaesthetized pigs. 2. Stimulation of filaments containing C heat nociceptor units gave small areas of elevated blood flow (average increase 96%, n = 11) restricted to the afferent receptive field. The extent of the areas of raised blood flow was imaged completely for 8 units. The average extent of vasodilatation in the direction of greatest spread was 8 mm and the maximum spread in any unit was 13 mm. 3. Stimulation of C polymodal nociceptor units never caused increases in blood flow in or near their receptive fields. 4. Localized noxious stimuli (55 degrees C or intradermal injection of capsaicin) caused flare extending 7-15 mm in the same skin region. 5. In agreement with the axon reflex model, spread of flare was restricted to the zone innervated by the terminals of single C fibre units. 6. It is concluded that the C heat nociceptor units are the major class of afferent involved in the flare reaction in the skin of the pig. C polymodal nociceptor units do not appear to be involved in flare in this species. The probable situation in human skin, which is also innervated by heat nociceptors, is discussed.