A total of 15 blue fox vixens aged 1-6 years were mated, 12 once on the first day of estrus and three a second time 48 hr after the first mating, and were killed 4 hr to 8 days following mating. Ova were collected from the oviducts, evaluated by stereomicroscopy, and studied by transmission (TEM; N = 49, 12 vixens) or scanning (SEM, N = 11, three vixens) electron microscopy. At 0-3 days after ovulation, the ova had not cleaved and were at different stages of meiotic maturation. In about one-half of these ova, representing all stages of meiotic maturation, a decondensing sperm head without nuclear envelope or a small pronucleus with partial nuclear envelope was observed. No clear relationship was found between maternal meiotic stage and the stage of paternal pronucleus formation. Sperm tails were never identified in the ooplasm. Cortical granules were released after sperm penetration at early stages of meiotic maturation. Thus the block against polyspermic penetration was activated during maturation of the oocyte. The first two-cell stage appeared 4 days after ovulation (3 days after mating), the first four-cell stage the following day (day 5), and the first eight-cell stage 6 days after ovulation (5 days after mating). In a single vixen mated late (7 days postovulation) two- to four-cell stages appeared the following day (day 8). This indicates that the time required for the first cleavage division decreases with increasing interval from ovulation to mating.(ABSTRACT TRUNCATED AT 250 WORDS)