Male mice have more cells that produce vasopressin (VP) in the bed nucleus of the stria terminalis (BNST) and medial amygdaloid nucleus (MeA) and denser VP-immunoreactive projections from these cells to the lateral septum and lateral habenular nucleus (LHb). Testosterone organizes this sex difference during the first week of life. Progesterone receptors (PR) may play a role in this differentiation. In rats, virtually all VP cells express PR, and progesterone reduces VP expression in the BNST and MeA and its projections (Auger et al., 2006). In addition, PR expression in the BNST is much higher in males than in females perinatally. We tested whether PR plays a role in sexual differentiation of VP expression by comparing wild-type and PR knock-out (PRKO) mice. In adulthood, all animals were gonadectomized and implanted with testosterone capsules. Three weeks later their brains were removed and processed for VP mRNA using in situ hybridization. Analysis of VP cell number in the BNST and MeA showed sex differences (males > females). Loss of PR did not change the size of the sex difference, suggesting that PR does not play a significant role in sexual differentiation of VP expression. As females had more VP mRNA signal per cell in the BNST than males the higher number of VP neurons in males is unlikely due to a difference in detectability of the cells. Analysis of the density of VP-immunoreactive projections mirrored the results on cell number with projections presumably coming from the BNST and MeA being denser in males than females. Projections presumably coming from the SCN showed a mixed pattern with projections to the medial preoptic area and principal nucleus of the BNST being denser in males than in females and projections in the medial dorsal nucleus of the hypothalamus not differing. Although the loss of PR did not affect sex differences in VP cell number, it increased VP fiber density in the LHb in both sexes.