The physiology of lactation in mice was studied in those aspects which were relevant to selecting mice for milk yield. The investigations fall under five headings.

Activities of lactating mice. From continuous records of the diurnal activities of mice, it was found that lactation causes no disarrangement of the mouse's day; both nursing and non -lactating females spend about 70% of their day on the nest, during which time the young are continually sucking; 20% of the day is spent at the food basket; and the remainder is divided equally between drinking and wandering about the cage. As her lactation advances, the female spends a smaller portion of each day on the nest, and the periods of nursing become shorter but more frequent. The lactating female spends no longer than the non-lactating female in feeding herself.

There is a slight rhythm, repeated daily, in the fraction of successive 4 hr. periods spent on the nest; the rhythm is the inverse of that for locomotory activity; and neither is as strong as the rhythm Bullough has described for locomotory activity in males.

Regulation of milk production according to litter size. Milk production is imperfectly proportional to the number of young suckled and is independent of the number of young born. Maximum amount of milk is available when the litter is born, and the supply is rapidly and permanently cut down to the requirements of the litter. The regulation is by partial involution of secreting tissue in all mammae.

Regulation presents difficulties in using litters of different sizes to measure lactation, and a method is given of constructing a chart whereby the relative performances of females suckling different sizes of litters can be compared.

Competition among suckling young. Although the mammary glands of any female are not equally developed, the apportionment of milk among her litter is usually fairly even, at least as far as this can be judged from the weights of her young at 12 days. This implies that young mice show no preference each for a particular teat and are constantly jostling each other for every teat. It seems, however, that when litter mates differ in the birth weights, in the unbalanced competition that results, the larger mice do get the greater share of milk. If the competition depended on any form of violence, it should be greatly reduced in small litters. Since there is no evidence of this, it is concluded that the competition is passive, i.e. that the larger young get the greater share of the milk because they feed faster, and not because they oust their weaker sibs from the best glands or spend longer sucking them.

Relations of lactation to body weight and age. Lactation performance of a mouse is correlated with her weight even when this is measured as early as 12 days of age. The correlation is no more pronounced when the measurement of weight is delayed until mating, i.e. only shortly before lactation. Evidence is given that different mechanisms are responsible for the correlations with earlier and with later weights. The earlier correlations demonstrate the inheritance of lactation performance from mother to daughter and the effect of the mother's lactation on her daughter's weight. This maternal effect on body weight diminishes in older mice, but inherent determination of weight increases. Thus the correlation of lactation with later weights arises from pleiotropy of gene action, and perhaps receives a contribution from effects of the environment that the two ‘characters’ have in common.

Lactation performance is also correlated with age, but only through the greater weight of older mice. Age seems to have no independent effect on lactation, e.g. on the development or efficiency of the mammary glands.