Rabbits were mated at timed intervals after an ovulationinducing injection of chorionic gonadotrophin. Blastocysts were recovered at autopsy 6 days after mating and examined cytogenetically for chromosome abnormalities. A marked decrease in the number of blastocysts recovered occurred at delayed ovulation-mating time intervals greater than 10 hr. Thirteen chromosome aberrations arose among 135 blastocysts examined. Triploidy was the commonest anomaly and was found in six instances from rabbits mated 8 and 9 hr after injection of gonadotrophin. Fewer abnormalities occurred at the remaining delayed intervals. In addition, a pentaploid, two mixoploids and a trisomy were obtained from animals mated immediately following injection.

Six-day blastocysts were recovered from rabbits mated at various intervals from 0 to 10 hours after an ovulation-inducing injection of chronic gonadotrophin. Ten of eighty or 13% of the blastocysts from animals mated between 6 and 9 hours after injection were triploid in chromosome constitution. When karyotyped, seven of the triploids had an XXY and three an XXX sex chromosome complex. The incidence of chromosome abnormalities at delay intervals from 0 to 4 hours was 5%, approximately. No triploids were found at these times. The sex chromosome complex of 6-day blastocysts was XY in 54% and XX in 46% of the blastocysts. No difference in the sex ratio was noted with delayed mating.

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