The effects of the timing of insemination relative to ovulation on fertilization rate, accessory sperm count and early embryo development were studied in sows. Oestrus detection was performed at intervals of 8 h. Sows were artificially inseminated once with 3 x 10(9) spermatozoa. Transrectal ultrasonography was performed at intervals of 4 h to determine when ovulation occurred and sows were killed at 120 +/- 6 h after ovulation. For each insemination-ovulation interval of 8 h, fertilization rates were as follows: > 48 h, 35% (n = 1); 48-40 h, 51 +/- 36% (n = 6); 40-32 h, 54 +/- 36% (n = 14); 32-24 h, 79 +/- 32% (n = 19); 24-16 h, 94 +/- 11% (n = 24); 16-8 h, 92 +/- 21% (n = 24); 8-0 h, 95 +/- 22% (n = 21) and for the sows that were inseminated after ovulation: 0 to -8 h, 75 +/- 38% (n = 26); -8 to -16 h, 74 +/- 43% (n = 15) and < -16 h, 0% (n = 1). The median accessory sperm count differed among the groups from 1 (insemination 40-48 h before ovulation) to 126 (insemination 0-8 h after ovulation) (P = 0.0001). Within each 8 h time interval, the normal embryos from sows with less than 90% normal embryos were less developed and had a lower sperm count than did the normal embryos from sows with more than 90% normal embryos (P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)