SUMMARY: Observations are recorded on the placenta of the guinea-pig at the 65th day of pregnancy. Placental weight is inversely related (a) to number of foetuses in the same horn of the uterus, and (b) to number of foetuses in the other horn. In litters with unequal numbers of foetuses in the two horns, placental weight is greater on the side with the smaller number. These observations are consistent with those previously recorded on foetal weight [Eckstein & McKeown, 1955]. Nevertheless, the variation in placental weight does not wholly account for the variation in foetal weight. Even when attention is restricted to foetuses with placentae of the same weight, foetal weight appears to reflect both a general effect, due to litter size and independent of distribution between the horns, and (less certainly, because of small numbers), a local effect, determined by contiguity of foetuses within the same horn. The possible significance of the last two observations is discussed.