1. 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.

2. Anatomical abnormalities of the reproductive organs, other than cystic ovaries, were found in sixteen animals (including two gilts) and the abnormality involved the ovary in eight of them. At least two of them were brought to notice because of the abnormalities so that the number seen affords a maximal estimate of the incidence of this type of aberration in the pig population from which the sample was drawn.

3. A wide variety of ovarian cysts was found, often within a single pair of ovaries. It is suggested that all the ‘types’ of cyst described here and elsewhere are different degrees of the same kind of aberration, the morphological form being determined by the stage in the ovulation process reached by the follicle (or corpus luteum) when overtaken by the physiological breakdown responsible for the cystic distension.

4. As none but slight degrees of cystic abnormality were found in pregnant animals, these were used as a criterion to distinguish between slight and severe degrees of cystic abnormality, the latter in all probability associated with sterility, and encountered in about 10% of the sows in the sample.

5. Slight degrees of cystic abnormality do not appear to be associated with the production of small litters; pregnancy appears to be either unaffected or else precluded altogether.

6. The reproductive organs of more than half of the sows which were discarded for reproductive failure were found to be normal, and in many cases the failure could only be attributed to chance. A large proportion of the sows discarded as sterile were culled after their first litter. The preponderance of reproductive failure at this stage could not be attributed to ovarian cysts.

7. There was a marked seasonal variation in the incidence of ovarian cysts, the proportion of animals with some degree of cystic abnormality being twice as high in the spring months as in the autumn. The variation is thought not to be due to sampling errors alone. There was no seasonal variation in the average number of corpora lutea or of embryos.

8. The observations are discussed in relation to work of a comparable nature done elsewhere, particularly in U.S.A. Some striking divergences are apparent. Large cysts, commonly found in pregnant animals in America, did not appear to interfere with gestation and were evidently formed during pregnancy. Such cysts were never observed in pregnant animals in the English material, where only nine out of 130 pregnant animals showed any degree at all of cystic abnormality, by no means severe in any of them, and not involving ‘large’ cysts. Marked enlargement of the clitoris was found to be associated with one type of ovarian cyst in America but was not observed in England. A greater diversity of cystic abnormality was recognized in the present work than in American studies, and such histological and endocrinological work as has so far been done has given results somewhat different from those recorded in America.

9. There is strong evidence that oestrus may fail to occur in the sow, so that the animal cannot be served. The condition is probably reported more frequently than it occurs, however, since reliance is often placed on examination alone for the detection of heat, without using a boar.

10. There is some evidence that oestrus and ovulation may get out of step in some animals and it is possible that the cause is related to that of ovarian cysts. Oestrus and service may occur during pregnancy.