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Forced swimming, as an effective stressor, has been found to facilitate the development of pair bonds in male but to interfere with this behaviour in female prairie voles (Microtus ochrogaster). In the present study, we found that forced swimming differentially influenced the expression of messenger RNA for vasopressin, oxytocin and corticotrophin-releasing hormone (CRH) in the paraventricular nucleus of the hypothalamus (PVN) in the prairie vole brain. Forced swimming did not alter vasopressin mRNA labelling, but did induce a sustained decrease in oxytocin mRNA labelling and a progressive increase in CRH mRNA labelling in the PVN. The elevated CRH mRNA labelling appeared to be due to an increased number of cells synthesizing CRH mRNA and an enhanced ability of individual cells to produce CRH mRNA. Male and female prairie voles did not differ in the vasopressin, oxytocin or CRH mRNA expression either at the basal levels or in response to swimming stress. Together, these data indicate that the hypothalamic response of vasopressin, oxytocin and CRH messenger RNAs to swimming stress is regulated by distinct transcriptional factors. In addition, it seems unlikely that these changes are involved directly in the sex differences in pair bond formation.