We studied the following properties of cat ventrolateral medullary (VLM) neurons that projected to the thoracic spinal cord: the relationship between their spontaneous activity and that in the inferior cardiac postganglionic sympathetic nerve, their responses to baroreceptor-reflex activation, their axonal conduction velocities, the funicular trajectories of their axons, the likely sites of termination of their axons, and their axonal branching patterns. Microstimulation in the second thoracic spinal segment (T2) antidromically activated 67 VLM neurons (as determined with time-controlled collision of spontaneous and evoked action potentials), whose activity was correlated to inferior cardiac sympathetic nerve discharge (as determined with spike-triggered averaging). We tested the effect of baroreceptor-reflex activation on the firing rate of 20 of these VLM-spinal neurons. Because the firing rate decreased in each instance, these neurons apparently subserved a sympathoexcitatory function. The axonal branching patterns of 51 VLM-spinal sympathoexcitatory neurons were studied. Thirty-four neurons were antidromically activated by stimulation in the T2 gray matter and in more caudal thoracic spinal segments (T11 and/or T6). In each case, the antidromic response evoked by stimulation in the T2 gray matter was due to activation of an axonal branch rather than the main axon (via current spread to the white matter). This was demonstrated with tests that included time-controlled collision of the action potentials initiated by stimulation in T2 and a more caudal thoracic spinal segment. Some VLM-spinal axons that projected to T11 branched in T6 as well as in T2. These data indicate that some VLM-spinal neurons exerted widespread excitatory influences on sympathetic outflow. Seventeen VLM sympathoexcitatory neurons that innervated the T2 gray matter could not be antidromically activated by stimulation in T5, T6, and T11 despite an extensive search at each level. Thus the axonal projections of some VLM-spinal neurons were restricted to upper thoracic segments. Antidromic mapping in T2 revealed that the axons of VLM sympathoexcitatory neurons coursed through the dorsolateral or ventrolateral funiculus to innervate the region of the intermediolateral nucleus. Mean axonal conduction velocity was 3.5 +/- 0.3 m/s. Those VLM-spinal axons restricted to upper thoracic segments generally were located dorsally and/or medially to those that innervated widely separated thoracic segments. The discharges of 35 other VLM neurons that were antidromically activated by T2 stimulation were not related to sympathetic nerve activity.(ABSTRACT TRUNCATED AT 400 WORDS)