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Observations are recorded on 65-day guinea-pig foetuses removed from twenty-nine experimental animals in which impregnation was limited to one horn of the uterus by transection of the other horn, and from twenty-nine unoperated control animals. The data are used to show that:

(1) As litter size increases, mean foetal weight and length decrease and total litter weight increases.

(2) Mean foetal weight and length decrease as the number of foetuses in the same horn increases, and as the number of foetuses in the other horn increases.

(3) In litters of less than four, association between rate of foetal growth and litter size is independent of distribution of foetuses between the two horns. In litters of four or more there is evidence of a local effect determined by the contiguity of foetuses within the same horn.

These observations suggest that the influence of litter size on rate of foetal growth is due partly to a local effect, determined by the number of foetuses within the same horn of the uterus, and partly to a general effect, determined by the total number of foetuses in the uterus, and independent of their distribution between the horns.

As the number of foetuses in one horn increases, mean number of foetuses in the other horn decreases. This result appears to be determined by a negative correlation between numbers of ova released from the two ovaries. It cannot be ascribed to differences in the proportion of corpora lutea represented by foetuses on the two sides.