Immunohistochemical methods were used to map the distribution of neurons exhibiting tyrosine hydroxylase-like immunoreactivity (TH) in the brainstem of the reptile Caiman crocodilus. The results reveal that many catecholamine systems previously described in mammalian and avian species are present in the brainstem of the caiman. Within the medulla, many immunoreactive neurons surround the central canal. This neuronal field extends rostrally to the level of the dorsal motor nucleus of the vagus. Many TH neurons overlap the region of the solitary nucleus, and an extensive system of fibers derived from these neurons extends ventrally and laterally into the region immediately bordering the descending nucleus of the trigeminal nerve. Some TH neurons are also present in the ventrolateral tegmentum of the medulla at this level. A large number of TH cells are present in the pons and midbrain. These include the locus coeruleus, nucleus subcoeruleus ventralis, nucleus subcoeruleus dorsalis, substantia nigra (Brauth et al., '83), and area ventralis of Tsai. The subcoeruleus nuclei are considerably larger in the caiman than in other reptilian species including turtles and lizards and closely resemble the subcoeruleus nuclei of birds in terms of position and anterior-posterior extent. Within the diencephalon, numerous small, intensely staining, TH-immunoreactive and CSF-contacting neurons were observed within the preoptic recess and in close proximity to the ventricular wall at rostral hypothalamic and preoptic levels. Many intensely stained, immunoreactive cell bodies were observed in the medial hypothalamus similar in position to the A13 cell group of mammals. In the subthalamus, TH neurons completely surround the ventral peduncle of the forebrain bundle (which contains fibers of the ansa lenticularis) and extend into the ventromedial and ventrolateral thalamic areas. A rich plexus of TH-positive axons and terminals invests the external layer of the median eminence.