Alterations in the opioid system in the hippocampal formation and some possible functional consequences were investigated in adult male rats that were prenatally exposed to either saline or morphine (10 mg/kg twice daily on gestational days 11-18) using in situ hybridization, Northern blots and autoradiography. Exposure of rats to morphine during early development alters the hippocampal opioid system, suggesting possible consequences for hippocampal-mediated functions