In this study, we investigated steroid regulation of the hyaluronan (HA) system in ovine endometrium including HA synthases (HAS), hyaluronidases, and HA receptor-CD44 using 30 adult Welsh Mountain ewes. Eight ewes were kept intact and synchronized to estrous (day 0). Intact ewes were killed on day 9 (luteal phase; LUT; nZ5) and day 16 (follicular phase; FOL; nZ3). The remaining ewes (nZ22) were ovariectomized and then treated (im) with vehicle (nZ6) or progesterone (nZ8) for 10 days, or estrogen and progesterone for

3 days followed by 7 days of progesterone alone (nZ8). Estradiol and progesterone concentrations in plasma correlated with the stage of estrous or steroid treatment. Our results showed trends (P! 0.1) and statistically significant effects (P! 0.05, by t-test) indicating that LUT had lower HAS1 and HAS2 and higher HAS3 and CD44 mRNA expression compared with FOL. This was reflected in immunostaining of the corresponding HAS proteins. Similarly, in ovariectomized ewes, progesterone decreased HAS1 and HAS2 and increased HAS3 and