The sexually dimorphic nucleus of the preoptic area (SDN-POA) in the rat represents a morphological substrate in which the influence of gonadal hormones on the process of sexual differentiation of the brain can be seen. Since the medial preoptic area (MPO) is a region rich in catecholamine (CA) terminals, it is possible that catecholamines may play a role either in the differentiation of the perinatal SDN-POA or in the function of this nucleus in the adult. It is not known whether catecholamine terminals exist within the SDN-POA or whether they can directly influence the activity of SDN-POA neurons. The present study was conducted to determine the extent to which catecholamines innervate this nucleus and further to elucidate the possibility of a potential sexual dimorphism in the innervation pattern. In order to determine which of the neurons in the MPO are within the SDN-POA we have utilized the fact that the SDN-POA has a prolonged period of neurogenesis in comparison to other neurons of the MPO. Thus, tritiated thymidine-labeled neurons can be used as a detection criterion for the SDN-POA. To conduct this experiment, timed pregnant Sprague-Dawley females were given a single injection of [3H]thymidine on Day 18 of gestation. Pups were killed as adults and prepared for fluorescence histochemistry of monoamines. Sections adjacent to those examined for catecholamine fluorescence were treated for autoradiographic localization of [3H]thymidine. Fluorescence innervation patterns were plotted within the boundaries of the nucleus following its identification from Nissl sections as well as from adjacent autoradiograms simultaneously viewed in a comparator bridge microscope with dark-field illumination.(ABSTRACT TRUNCATED AT 250 WORDS)