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The growth and food consumption of mature female rats and of lactating rats and their litters reared under various conditions of illumination have been measured.

Mature female rats grew less when kept in the dark than in the light.

Newborn rats reared in total darkness by does which were allowed in the light for only 1 hr. daily grew less during the first 18 days of life than their litter-mate controls.

Newborn rats reared under natural lighting by does with both eyes removed did not grow as well as litter-mates reared by does with one eye removed.

The mean weight of food eaten daily by mature female rats when in the dark did not differ significantly from that eaten when in natural lighting. Lactating does and their litters confined in total darkness and bilaterally enucleated does ate significantly less food than their controls. The reduction in food intake was apparent by the third day after parturition. On the other hand, when the doe received 1 hr. illumination daily, the daily food intake of the experimental litters was depressed only during the latter part of lactation.

The discussion suggests a possible way in which the effects of darkness on the normal adult female rat and on the lactating rat may be interrelated.