Production of hyaluronan (hyaluronic acid: HA) was demonstrated in denuded mouse oocytes (DOs) by the enzyme-linked immunosorbent assay, and the role of HA in enlargement of the perivitelline space in the oocytes was examined. The incidence of polyspermy following insemination was also observed in DOs in which HA synthesis was inhibited. HA was not detected in culture medium containing DOs immediately after collection. After culture for 7 h, 4.75 pg of HA per DO was detected in the medium, and the mean amount of HA significantly increased to 20.78 pg 14 h after culture. When DOs were cultured in medium containing 0.25 mM 4-methylumbelliferone (MU), an inhibitor of HA synthase, the mean amount of HA in the culture medium with DOs was 8.61 pg, which was significantly smaller than the amount in the control medium with non-treated DOs (21.59 pg). The mean size of the perivitelline space in oocytes cultured with cumulus cells (5.40 μm) did not differ from that (5.08 μm) of DOs. The mean size of the perivitelline space was significantly smaller in the MU-treated DOs (3.58 μm) than in the control DOs (4.65 μm). The fertilization rate did not differ between the MU-treated DOs (84.9%) and control DOs (81.0%), whereas the incidence of polyspermy was significantly higher in the MU-treated DOs (13.3%) compared with the control DOs (2.1%). These findings clarified that the HA involved in enlargement of the perivitelline space in oocytes is synthesized and secreted by the oocytes themselves. They also suggest that there is a close relationship between the size of perivitelline space and the incidence of polyspermy in mouse oocytes.