1. Individual neurones in the guinea-pig superior cervical ganglion were studied to determine whether they are innervated by preganglionic axons with similar conduction velocities. 2. Latencies of synaptic responses recorded intracellularly in ganglion cells after stimulation of individual ventral roots varied from 28 to 430 msec. Most of this variability arose from differences in preganglionic conduction velocity. 3. The twelve different axons that on average innervate each ganglion cell tended to have broadly similar conduction velocities; a neurone receiving a rapidly conducting input was usually contacted by other rapidly conducting axons, and vice versa. 4. Preferential innervation of individual neurones by axons with similar conduction velocities was evident even when only axons arising from the same spinal segment were compared. Thus preferential innervation by axons of similar conduction velocity cannot be simply a manifestation of segmental preferences. 5. These results suggest that the mature pattern of innervation in mammalian sympathetic ganglia reflects the functional as well as the positional qualities of the synaptic partners.