This study examined the effects of extending oocyte maturation 4 h beyond current methods and capacitating sperm with or without heparin 4 h before oocyte introduction to determine whether embryo development would increase after in vitro fertilization. Oocytes were aspirated from ovaries that were collected at slaughter. Cumulus-enclosed oocytes were matured in M-199 supplemented with serum from cows in standing estrus (20%), antibiotic-antimycotic solution (1%), HEPES (10 mM), and equine LH (30 micrograms/ml) in a humidified 5% CO2 atmosphere. Oocytes were matured for either 24 or 28 h and subsequently fertilized with sperm that had been capacitated 0 or 4 h (before oocyte contact) with or without heparin (0.2 microgram/ml). Data were analyzed as a 2 x 2 x 2 factorial design. Percentages of cleavage and development were transformed by the arcsin square root method before analysis of variance. An interaction of maturation length and sperm capacitation resulted because cleavage rate decreased with precapacitated sperm, but only within the 24-h maturation period. Heparin increased cleavage rate at 48 h after fertilization but did not affect further development. More oocytes developed to morulae when they matured for 24 h than when they matured for 28 h. In conclusion, a 24-h maturation length without precapacitated sperm was optimal for the subsequent development of cumulus-oocyte complexes.