Comparisons of high- and low-affinity [3H]imipramine binding to whole brain homogenates from adult male and female rats of the Fawn-Hooded and Long-Evans strains were performed. Most strikingly, no significant differences were observed between the two strains in any of the binding parameters, indicating that brain [3H]imipramine binding sites, which may be related to the serotonergic uptake process, appear normal in a strain of rats with serotonin platelet storage pool disease. However, a significant sex difference in high- but not low-affinity whole brain [3H]imipramine Bmax values was observed, with females of both strains having higher densities than males. The observed sex difference in densities of high-affinity [3H]imipramine binding sites necessitates further research into possible sex hormone interactions with this binding site and serotonergic transmitter systems.