Primer oocytes (n=520) collected from ovaries of slaughtered cows were divided into 3 groups, and matured at 39 degrees C with 95-100% humidity and under a gas mixture of 5% O-2, 5% CO2 and 90% N-2 for 22, 24 and 26 hours. Modified Parker's Medium (MPM) supplemented with FSH and 20% ECS was used as maturation medium. At the end of these periods, oocytes (22 h.=122, 24 h.=143 and 26 h.=255) were taken into Tyrode-lactate medium supplemented with BSA, heparin and PHE, and frozen-thawed bull semen was add after swim-up procedure (10 mu congruent to 400.000 spermatozoa/30-35 oocytes). Oocytes matured in 3 groups were kept with spermatozoa for 14, 16, 18, 20, 22 and 24 hours. At the end of these time periods all oocytes were fixed, stained and examined by phase-contrast microscope.