ABSTRACT 863 female pigs, mostly sows discarded from commercial herds in East Anglia and sold for slaughter, were examined when killed and the condition of the reproductive tract was related to the animal’s reproductive history when it was known. The most frequent number of corpora lutea was 20, the average 19·6. Nineteen pigs pregnant for the first time (gilts) had an average of 13·6 corpora lutea and 9·8 embryos; 83 in their second or subsequent pregnancies (sows) averaged 19·25 corpora lutea and 11·9 embryos. The average number of corpora lutea (ovulations) rose sharply after the first litter, continued to rise slightly until after the fourth litter, and rose again after the fifth. The initial rise and subsequent upward trend for several litters is similar to what has been recorded elsewhere for the litter size at birth, but the increasing number of ovulations in later life is in contrast with the falling off which has been observed in the size of the litter at birth. If both occur in the same population, as seems likely, there must be a substantial increase in embryonic mortality in the later litters. The percentage of ova lost between ovulation and the time of observation was greater in parous than in nulliparous animals, but the difference was found to be attributable to the greater initial size of litters in sows, large litters being found to lose proportionately more ova than small ones. Five of 19 gilts and 4 of 83 sows showed no loss of ova. At least 40 per cent, of all ova ovulated are lost before parturition. About 85 per cent, of the loss is incurred in the first half of gestation, and something like three-quarters of it is incurred before the 25th day of pregnancy. It is suggested that most of the embryonic loss probably occurs at or about the time of implantation between the 10th and 20th day. Even in the case of the largest litters there is no evidence of heavy loss in late stages of pregnancy that could be attributed to the sow’s inability to accommodate or nourish the foetuses.