This dissertation is an effort to further define the central relationship of oxytocin and relaxin. The enclosed experiments have used rats and pigs to study this relationship;The first study looked at the effect of relaxin on the secretion of oxytocin and prolactin in pregnant gilts. Intravenous administration of relaxin appeared to mildly increase the circulating oxytocin and prolactin in pigs at midgestation, but had no effect in late gestational gilts. However, removal of endogenous relaxin in late gestational gilts did cause a large secretion of oxytocin and a mild release of prolactin, followed by a rapid return to baseline values for both hormones. This may indicate that relaxin acts to prevent the premature release of oxytocin in late gestation;The second study looked at the effect of relaxin on the activity of oxytocinergic cells in the hypothalamus of late pregnant rats following intravenous administration of relaxin. Cellular activation was visualized using the immunohistochemical labeling of Fos as a marker. In animals which received relaxin, Fos-like immunoreactivity was increased over that in animals which received vehicle. The increased Fos-like immunoreactivity was found in the supraoptic nucleus (SON), paraventricular nucleus (PVN) and subfornical organ. The Fos-like immunoreactivity in the SON and PVN was co-localized in cells exhibiting oxytocin-like immunoreactivity. This suggests that relaxin is changing the cellular activity within oxytocinergic cells;The third study goes on the determine if the activity seen following relaxin administration within oxytocinergic cells of late pregnant rats, is an increase in the synthesis of oxytocin. In situ hybridization was used to compare levels of oxytocin mRNA within the SON and PVN between rats exposed to endogenous or exogenous relaxin and those with no circulating relaxin. No differences were seen in the amount of oxytocin mRNA present as a result of the presence or absence of relaxin in the circulation;Collectively, the results from these studies indicate that relaxin affects the release of oxytocin during pregnancy. Relaxin probably does this by changing activity at the level of the cell body, but the rate of synthesis of oxytocin is unchanged.