To characterize the fine morphology of individual reticulospinal axons at multiple spinal segments, localized injections of the anterograde neural tracer, Phaseolus vulgaris leucoagglutinin (PHA-L), were made into the nucleus reticularis pontis oralis (NRPo) of the cat. Following survival periods of 6-8 weeks, labelled axons, between 1 and 8 microns in diameter, were found throughout the cervical and upper thoracic segments. Thick axons (diameter > or = 3 microns) were found to descend beyond the upper thoracic spinal cord, while most thin axons (diameter < 3 microns) ended in the upper cervical cord. From serial transverse sections (50 microns) of segments C3 to T2, in four cats, the trajectories of 23 single, thick reticulospinal axons were traced in continuity over distances of between 21.8 and 59.4 mm, corresponding to 3 and 8 segments, respectively. Most axons gave off at least one, and as many as four collaterals per segment, some preferentially in the cervical enlargement. The remainder gave off collaterals at most but not all segments. Detailed reconstruction of the collateralization and arborization in the spinal gray matter showed two major termination types, one where terminals remained ipsilateral to the stem axon, the other where additional collaterals extended across the midline from the ipsilateral gray matter to terminate in the contralateral gray matter. Axons tended to have collaterals of one type or the other, irrespective of the rostrocaudal level. Both ipsilateral and bilateral projections terminated mainly in laminae VII or VIII although the branching patterns varied from axon to axon. Individual stem axons, in general, showed similar termination patterns at each level.