Pituitary levels of prolactin were significantly diminished after 5 s.c. injections spaced 12 h apart of 5 μg of a partially purified bovine pineal peptide fraction (E5) in both intact and castrated rats. E5treatment did not significantly affect the castration-induced changes in plasma luteinizing hormone (LH), follicle-stimulating hormone (FSH) or pituitary LH but did partially block the pituitary fall in FSH in castrated animals.