We have investigated the organization of the guinea-pig thoracic chain by studying the innervation of the stellate and fifth thoracic sympathetic ganglia with intracellular recording. 1. These ganglia receive preganglionic innervation from different but overlapping sets of spinal cord segments: the stellate ganglion is innervated by preganglionic axons from spinal segments more rostral than those supplying the fifth thoracic ganglion, but somewhat more caudal than those innervating the superior cervical ganglion. 2. Individual thoracic ganglion cells are innervated by only some of the spinal segments that supply each ganglion as a whole. In general, the subset of spinal segments innervating a ganglion cell is contiguous; one of these segments provides the strongest innervation, with progressively weaker innervation arising from spinal levels adjacent to the dominant one. This selective pattern of innervation is similar to that in the superior cervical ganglion (Njå & Purves, 1977 a). 3. Preganglionic axons frequently innervate neurones in more than one ganglion. 4. Although neurones innervated by the same spinal cord segments are found in both the stellate and the fifth thoracic ganglion, as well as in the superior cervical, the number of ganglion cells receiving innervation from particular spinal segments is different in each ganglion. Moreover, neurones dominated by the same segment but located in different ganglia receive somewhat different average innervation from adjacent segments as a function of the ganglion in which they reside. 5. These results indicate that neurones in the thoracic chain ganglia, as those in the superior cervical ganglion, are selectively innervated by particular spinal cord segments. We suggest that the different average innervation of sympathetic ganglia reflects at least two related factors: the selective qualities of their constituent neurones, and the availability of different preganglionic axons to each ganglion.