The purpose of this study was to determine the amount of milk and stock diet consumed by rats during postnatal development and the extent to which they feed in circadian rhythms when weaned naturally by their dams or when separated from their dams at age 21 days. One approach used in several experiments was to record feed bin weight losses at circadian intervals. The second approach was to analyze the stomach contents of rats killed at circadian intervals at representative ages. Contents were inspected, freeze-dried, weighed and assayed for fat by extraction with solvent. The difference in fat between rat milk solids and stock diet was the basis for estimating proportions of stock diet and milk. The results indicate that 1) natural weaning occurs progressively from 14 to 30 days after birth, with an accelerated phase between 18 and 25 days; 2) pups up to 8 days old and their dams feed mostly by light and darkness, respectively; 3) circadian variations in feeding are weak and inconsistent in 10 to 20 day old pups and their dams; 4) at 21 days pups and dams begin to feed mostly in darkness; 5) the proportions of food consumed by light and darkness approach those of the adult by age 29 days when rats are weaned naturally, and by 25 days when weaned at 21 days; and 6) progression into the nocturnal feeding pattern may be disturbed if larger litters are not provided with sufficient space.