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.