The distribution of dopamine (DA) immunoreactivity in the forebrain and midbrain of the red-eared turtle, Pseudemys scripta elegans, was studied using recently developed antibodies against DA. DA-containing cells were found around the glomeruli of the olfactory bulb but not in the telencephalon proper. In the diencephalon DA cells were observed in the preoptic region, several parts of the periventricular hypothalamic nucleus, the periventricular organ, the ependymal wall of the infundibular recess, the lateral hypothalamic area and the pretectal posterodorsal nucleus. In the midbrain DA cells were found in the ventral tegmental area, the substantia nigra and the presumed reptilian homologue of the mammalian A8 cell group. Dopaminergic fibers and terminals were observed throughout the whole brain, particularly in the telencephalon and diencephalon. The olfactory tubercle, the striatum and the nucleus accumbens appear to have the most dense innervation, but the anterior olfactory nucleus and the septal area also show numerous DA fibers and terminals. Cortical areas are in general not densely innervated by DA fibers. Compared to the results obtained for a lizard, Gekko gecko, with the same antibodies, the results of the present study are very similar as regards the distribution of DA neurons, fibers and terminals. In having better developed DA cell groups in the midbrain and a stronger innervation of the striatum, Pseudemys resembles mammals more than does Gekko. In contrast, the many cerebrospinal fluid-contacting DA neurons in the hypothalamus of Pseudemys are a primitive feature of the diencephalon. The previous immunohistochemical study of Gekko, a lizard, and the present account of Pseudemys, a turtle, indicate that at least two different lines of evolution exist within the reptiles with regard to the DA innervation of the dorsal ventricular ridge. One, including turtles and, probably, crocodilians with a weak DA innervation; and another, represented by lizards, with a strong DA immunoreactivity.