This paper describes an analysis of the first cell cycle of mouse oocytes aged postovulation and fertilized in vivo. For this purpose, we developed a procedure for inducing ovulation in vivo that allows accurate timing of ovulation. The method is based on a luteinizing hormone (LH)-releasing hormone (LHRH) administration at prcestrus. This ovulation procedure had no detectable effect on the rate of ovulation or postimplantation embryonic death. We used this method of ovulation induction in an analysis of the separate stages of the first cell cycle of in vivo fertilized postovulation aged oocytes. All stages assessed were shorter in aged oocytes (12 hr postovulation) than in zygotes from unaged oocytes (1 hr postovulation): (1) the time interval between insemination and penetration of the aged oocytes was 1.5 hr shorter than the time interval of the unaged oocytes; (2) pronuclear formation in the fertilized aged oocytes was somewhat quicker than pronuclear formation in fertilized unaged oocytes; (3) in zygotes from aged oocytes, the time between formation of pronuclei and the pronuclear membrane breakdown was 1 hr shorter than in zygotes from unaged oocytes; (4) the first cleavage division was 3 hr advanced in zygotes from aged oocytes compared with the moment of the first cleavage division in zygotes from unaged oocytes. We also determined the glutathione (GSH) content of unaged and aged oocytes to investigate a possible relationship between the rate of pronuclear formation and GSH. The level of GSH was two times lower in oocytes aged postovulation for 12 hr than in unaged oocytes. The level of GSH in fertilized, unaged oocytes was half that in unfertilized, unaged oocytes; this decline was not observed after fertilization of aged oocytes, however. In summary, we developed a procedure that allows in vivo fertilization at defined points of time after LHRH-induced ovulation. Under the conditions used in this procedure, we could detect several differences between zygotes derived from unaged and postovulation aged oocytes.