This study was conducted to evaluate morphologic differences in pig oocytes matured in vivo and in vitro, with particular reference to the potential relationship between oocyte morphology and the occurrence of polyspermy after in vitro fertilization (IVF). In vivo-matured oocytes were surgically recovered from the oviducts of gilts with ovulated follicles on day 2 of estrus, and in vitro-matured oocytes were obtained by culturing follicular oocytes in a oocyte maturation system that has resulted previously in production of live offspring following IVF. Comparisons were made of the cytoplasm density, the diameter of oocytes with or without zona pellucida (ZP), the thickness of the ZP, the size of the perivitelline space (PVS), ZP dissolution time, and cortical granule (CG) distribution before IVF, and CG exocytosis and polyspermic penetration after IVF. Oviductal oocytes have clear areas in the cytoplasm cortex, while in vitro-matured oocytes have very dense cortex. The diameter of ovulated oocytes with ZPs was significantly (P < 0.001) greater than that of in vitro-matured oocytes. However, no difference was observed in the diameter of the oocyte proper. Significantly (P < 0.001) thicker ZPs and wider PVSs were observed in the ovulated oocytes. The ZPs of ovulated oocytes were not dissolved by exposure to 0.1% pronase within 2 hr, but the ZPs of in vitro-matured oocytes were dissolved within 131.7 +/- 7.6 sec. The ZPs of ovulated oocytes, but not of in vitro-matured oocytes, were strongly labeled by a lectin from archis hypogaea that is specific for beta-D-Gal(1-3)-D-GalNAc. Polyspermy rate was significantly (P < 0.01) higher for in vitro-matured oocytes (65%) than for ovulated oocytes (28%). CGs of oviductal oocytes appeared more aggregated than those of in vitro-matured oocytes. Most of CGs were released from both groups of oocytes 6 hr after IVF regardless of whether they were polyspermic or monospermic oocytes. These results indicate that in vitro-matured and in vivo-matured pig oocytes possess equal ability to release CGs on sperm penetration. Unknown changes in the extracellular matrix and/or cytoplasm of the oocytes while in the oviduct may play an important role(s) in the establishment of a functional black to polyspermy in pig oocytes.