The course and terminations of the spinal and supraspinal projections of rat dorsal horn lamina I cells have been determined by antidromically activating the cells with a roving stimulating microelectrode which was used to map systematically the brainstem of the animals. The cells studied are the most common type of projection cell in lamina I of the rat with ascending axons coursing in the contralateral dorsolateral funiculus at C2. In the present study we have found that the axons decussate within 1-5 mm of the cell body. The ascending axons occupy a peripheral site within the dorsolateral funiculus at C2. Within the brainstem it was possible to identify a main or 'parent' axon which showed a progressive drop in conduction velocity as it coursed rostrally. All these terminated in the midbrain. In addition to the main branch each axon gave rise to several collaterals and these terminated in: ventrolateral periaqueductal grey and the immediately lateral n. cuneiformis; and the dorsal medullary reticular formation. (i.e. most fibres had two different projection zones). Since these cells did not project to the thalamus, it may be that the information they carry is used other than for purely sensory processes. One possibility is that they impinge on an antinociceptive system believed to originate in the periaqueductal grey.