Adult hypophysectomized FSH-primed mice were used to study ovulation, fertilization, and preimplantation embryo development. Twelve days after hypophysectomy, animals were injected sc with oFSH (4 micrograms/day) twice a day for 3 days. This resulted in large preovulatory follicles that did not secrete estrogen. Concurrent with the last FSH injection, either hCG (5 IU) or human recombinant FSH (10 IU) was injected sc to induce ovulation. Animals were mated or not and killed 1-4 days later. The ovulation rate was similar for both the hCG-induced group (FH) and the FSH-induced group (FF), 97% and 90%, respectively. About 45% of the FH mice mated successfully with 56% of the eggs fertilized compared to only 25% of the FF mice with 45% of the eggs fertilized. However, only 5% of ovulated eggs developed to four-cell stages in vivo by day 3 for the FH animals and none in the FF group. To determine the reasons for the in vivo retarded embryo development, embryos at the one- or two-cell stage were collected on day 2 from the FH group. After 96 h of culture, 22% of two-cell embryos were converted to blastocysts, and 11% of one-cell eggs divided to the four-cell stage. In contrast, 80% of two-cell embryos from normal mice develop into blastocysts by 72 h of culture. The ovarian incubation medium and serum were used to measure progesterone (P4), androstenedione (A), and estradiol (E2). The patterns of serum and in vitro production of steroids were parallel. In FH mice, P4 increased immediately on the day after hCG injection (day 1) and decreased progressively on days 2 and 3; A and E2 levels increased on day 2, A decreased on day 3, and E2 decreased on day 4. When human recombinant FSH was used to induce ovulation, there were no significant changes in serum P4 and A; E2 levels were about 4 times higher on day 1 than in the FSH-primed control, then dropped to baseline levels on days 2 and 3. However, on day 3 in both the FH and FF groups, FSH receptors were still present on the granulosa cells of antral follicles, and LH/hCG receptors were present on the granulosa cells of large antral follicles and newly formed corpora lutea.(ABSTRACT TRUNCATED AT 400 WORDS)