Species differences in the distribution of beta-adrenergic receptors in the hippocampal and retrohippocampal regions of rats and guinea pigs were examined using in vitro autoradiographic techniques. beta 1-receptors were found in the hippocampal area CA1 of both species, although guinea pigs had significantly lower receptor densities in comparison to rats. In guinea pigs, beta 2-adrenergic receptors were predominant in hippocampal area CA1. Hippocampal area CA3 had very low levels of beta 1- and beta 2-receptors in both species. The retrohippocampal area was also found to have a distinct topographic distribution of beta-receptors. In rats, the subiculum and parasubiculum (layers II-III) were heavily labeled for beta 1-receptors; in contrast, guinea pigs had few receptors in these regions. beta 2-receptors were particularly prominent in the parasubicular region in rats. The entorhinal cortex laminae was found to contain beta-receptors in both rats and guinea pigs. Immunohistochemical techniques were used to compare the pattern of catecholaminergic innervation with the receptor distribution within each hippocampal subregion. Despite the general lack of beta-receptors in area CA3, abundant catecholamine immunoreactive fibers were observed in CA3 of rat and guinea pig hippocampus. Significant species differences were found in the distribution of hippocampal beta-adrenergic receptor subtypes, and moreover, in both species the distribution of beta-adrenergic receptors did not coincide with the pattern of hippocampal adrenergic innervation.