Preadipocyte factor-1 (Pref-1)/delta-like protein/fetal antigen-1 (FA1) is a member of the epidermal growth factor-like family. It is widely expressed in embryonic tissues, whereas in adults it is confined to the adrenal gland, the anterior pituitary, the endocrine pancreas, the testis and the ovaries. We have previously cloned Pref-1 from neonatal rat islets stimulated by GH. The aim of the present study was to elucidate the biosynthesis and release of Pref-1/FA1 in beta-cells and to determine if Pref-1/FA1 is mediating the mitogenic effect of GH in insulin-producing cells. First we studied the biosynthesis and processing of Pref-1 to the soluble form, FA1, in pancreatic islets and insulinoma cells transfected with Pref-1 cDNA. We measured the release of FA1 by ELISA and the possible effect of FA1 in GH-stimulated beta-cell proliferation by incorporation of bromodeoxyuridine (BrdU) in insulin-positive islet cells. We found that Pref-1 was synthesized in normal islets and in RINm5F insulinoma cells and released into the medium in two forms, of which one corresponded to FA1. Both the expression of the mRNA for Pref-1 and the release of the soluble form(s) were stimulated by GH and prolactin (PRL). Whereas 2 h exposure to high glucose or 3-isobutyl-1-methylxanthine stimulated insulin release, only a small change was seen in FA1 release, suggesting that the FA1 is released by a different pathway than insulin. However, long-term exposure (48 h) to high glucose increased FA1 secretion, indicating that FA1 is regulated by glucose. Neither FA1 nor conditioned medium from GH-stimulated islets depleted for GH was able to increase beta-cell replication and overexpression of Pref-1 resulted in attenuated proliferation of the RINm5F cells. By immunocytochemistry of GH-stimulated islet cells no correlation between high Pref-1 expression and BrdU incorporation was observed and there was an inverse relationship between the levels of insulin and Pref-1. These results indicate that Pref-1/FA1 is not mediating the mitogenic effect of GH and PRL. Therefore the function of Pref-1 in the beta-cell remains unknown.