AbstractThese studies reveal that the postnatal ontogeny of serotonin (5HT) in the sympathetic nuclei of the rat spinal cord is protracted; the adult complement of 5HT‐immunoreactive fibers is not achieved until at least 60 days of age. As descending serotonin fibers innervate and demarcate the distribution of preganglionic sympathetic nuclei, rostral‐caudal and temporal gradients exist. Additionally, a heterogeneous segmental 5HT ontogenetic pattern is observed in sympathetic nuclei. Most serotonin fibers in laminae VII and X are unorganized at birth except for some sympathetic nuclei in high thoracic regions where the 5HT sympathetic pattern is being initiated. By postnatal day 6 the framework of the 5HT pattern is established in all sympathetic nuclei, and by postnatal day 16 a pattern is formed, which develops into the compact adult state by postnatal day 60. The protracted period of sympathetic 5HT development corresponds with the length of time it takes for the autonomic nervous system to mature. In addition, 5HT intraspinal cell bodies are observed at all time points examined, except for the day of birth, and are found in the same regions as adult 5HT neurons, i.e., dorsal or lateral to the central canal in laminae VII and X and in all spinal segments except cervical levels. Many of the 5HT neurons are pericanalicular and bipolar in appearance. Multipolar 5HT neurons are first observed on postnatal day