Few embryos derived from pig oocytes matured and inseminated in vitro are able to develop to blastocyts in culture. The present study was conducted to examine the effects of oocyte maturation media on the developmental ability of pig oocytes matured and inseminated in vitro. Follicular oocytes collected from ovaries of prepubertal gilts were cultured in NCSU23 medium, tissue culture medium 199 or a modified Whitten's medium. All of the media were supplemented with 0.57 mmol cysteine l-1 and 10% pig follicular fluid. After maturation, some of the oocytes were used for examination of intracellular glutathione content, nuclear maturation and cortical granule distribution. The other oocytes were inseminated in vitro in a modified Tris-buffered medium with cryopreserved, ejaculated spermatozoa for examination of cortical reaction, sperm penetration, male pronuclear formation and blastocyst development. No differences (P > 0.05) were observed in nuclear maturation, cortical granule distribution, sperm penetration, male pronuclear formation, polyspermy and cleavage in oocytes matured in the three media. However, significant (P < 0.05) differences were observed in glutathione content, cortical granule exocytosis, blastocyst development and number of cells in blastocysts. NCSU23 medium gave the best results of the three media, resulting in 5.8 pmol glutathione per oocyte, 97% of cortical granule exocytosis, 30% blastocyst development and 36.8 +/- 17.0 cells per blastocyst. These results clearly indicate that cytoplasmic maturation of pig oocytes was significantly affected by oocyte maturation media even in the presence of cysteine and pig follicular fluid. In addition, it was demonstrated that a large proportion of pig oocytes can develop to blastocysts under in vitro conditions.