Insulin-like growth factor II (IGF-II) is expressed and secreted by the choroid plexus and has been suggested to act as a trophic factor in the adult mammalian central nervous system. The aim of the present study was to investigate whether IGF-II has an autocrine role in the choroid plexus. Using in situ hybridization we demonstrate that IGF-II is primarily expressed in the epithelium of adult rat choroid plexus. Conditioned medium from primary cultures of purified rat choroid plexus epithelial cells, intact choroid plexus tissue, as well as rat CSF, displaced IGF-II binding to a 23 HMM melanoma cell line in an IGF-II radioreceptor assay. The presence of IGF-II and IGF binding protein-2 in conditioned medium was shown by Western immunoblot. The mitotic activity in choroid plexus epithelial cell cultures was quantified by immunohistochemical staining of bromodeoxyuridine incorporated into cell nuclei. A monoclonal antibody towards IGF-II inhibited cell division by 35%, while IGF-I increased the number of stained nuclei by 75%. Basic fibroblast growth factor stimulated cell division at low concentrations, but had no effect at high concentrations. Growth hormone had no effect. We conclude that IGF-II in the choroid plexus could have an autocrine role in the regulation of choroid plexus epithelial cell growth.