Daily rhythms of secretion have been described for luteinizing hormone (LH) and prolactin (PRL) from the anterior pituitary of rats. Using selective opioid antagonists, we found that μ andxopioid receptor ligands regulate LH and PRL secretion and, of particular interest, that the magnitude of opioidergic effects varies with the time of day. In addition, incomplete temporal overlapping of the LH and PRL responses to the antagonists suggests that different endogenous opioid pathways, with different temporal profiles of peptide release, may control each of these hormones.— Almeida, O. F. X.; Nikolarakis, K. E.; Webley, G. E.; Herz, A. Opioid components of the clockwork that governs luteinizing hormone and prolactin release in male rats.FASEB J.2: 2874‐2877; 1988.