Our observation of galanin-immunoreactive perikarya with morphological characteristics strikingly similar to those containing gorvadotrophin-releasmg hormone (GnRH) led us to undertake double-label immunohistochemical studies on the distribution of these peptides. A considerable proportion of GnRH-containing perikarya in the rat preoptic area were found to be immunoreactive for galanin. In order to establish whether this coexistence contributes to the control of luteinizing hormone (LH) release, the two peptides have been perfused alone and in combination through anterior pituitary cell columns derived from female rats in various endocrine conditions. Galanin at 0.1 mu M had a small stimulatory effect on LH release from cells obtained on the day of pro-oestrus; this effect was less than that obtained with GnRH at 4 nM. Cells from oestrous rats showed no LH release in response to galanin. The studies undertaken thusfar have not indicated that galanin has a priming effect on the release of LH induced by subsequent treatment with galanin or a potentiating effect when administered together with GnRH. Delta sleep-inducing peptide, another neuropeptide recently reported to be coexistent with GnRH, was also found to be capable of inducing a small discharge of LH from anterior pituitary cells obtained on the day of pro-oestrus. Further studies will be required to establish the full consequences of the coexistence of these peptides and GnRH.