The sites of localization of luteinizing hormone (LH), follicle-stimulating hormone (FSH), growth hormone (GH), adrenocorticotrophic hormone (ACTH) and prolactin (PRL) within placental tissues have been studied by an immunoperoxidase technique. The syncytiotrophoblast is the sole significant site of localization of LH, FSH, GH and ACTH; PRL is found both in syncytiotrophoblast and in decidual cells. It is highly probable that the sites of localization of these peptide hormones represents their sites of synthesis in the placenta and thus that the syncytiotrophoblast is the sole site of synthesis of LH, FSH, LH and ACTH. PRL appears to be synthesized both in syncytiotrophoblast and decidua, but the latter is probably not the major site of synthesis of this hormone. Whether these placental peptide hormones have any physiological role to play during pregnancy or whether the placental capacity to synthesize such hormones is an atavistic phenomenon of no functional importance is currently a moot point.