1. Extracellular recording techniques were used to study nerve terminal impulses (NTIs) recorded from single polymodal nociceptors and cold-sensitive receptors in guinea-pig cornea isolated in vitro. 2. The amplitude and time course of NTIs recorded from polymodal nociceptors was different from those of cold-sensitive receptors. 3. Bath application of tetrodotoxin (1 microM) changed the time course of spontaneous NTIs recorded from both polymodal and cold-sensitive receptors. 4. Bath application of lignocaine (lidocaine; 1-5 mM) abolished all electrical activity. 5. Local application of lignocaine (2.5 and 20 mM) through the recording electrode changed the time course of the NTIs recorded from polymodal nociceptors but not that of NTIs recorded from cold-sensitive nerve endings. 6. It is concluded that action potentials propagate actively in the sensory nerve endings of polymodal nociceptors. In contrast, cold-sensitive receptor nerve endings appear to be passively invaded from a point more proximal in the axon where the action potential can fail or be initiated.