Changes in composition of rat's milk in the stomach of the suckling.

I. A study was made of the partition of the major constituents of rat’s milk during the formation of the clot in the infant rat’s stomach. 2. Milk was obtained from the dams by manual expression after separation from their litters for periods ranging from 12 to 23 h. 3. The ratio of fat to protein in the clots removed from the stomachs of the 11-day-old pups was more than double that found in the milk. The difference was due to the high fat content of the clot. A. The stomach contents of DUDS which were allowed to suckle for I h following a 12 h -~~ separation from their dams had ;fa; to protein ratio similar to that of the expressed milk, When suckling was prolonged, the ratio reverted to that of the established clot. 5 . Milk secreted under conditions of uninterrupted lactation and promptly recovered from the stomachs of pups previously fasted had a value for fat to protein comparable to that for the clot from a normally fed pup. An interruption in suckling therefore reduced the fat content of the milk. 6. Part of the carbohydrate in rat’s milk is secreted in a bound form. Lactose accounts for approximately one half of the total carbohydrate. Evidence is presented which suggests that the remainder may be an isomer of lactose. 7. Fat and lactose undergo hydrolysis to a small extent only in the pup’s stomach. The breakdown of protein is insignificant. Rat’s milk contains no proteolytic enzymes. 8. No significant differences in composition were found in the outer shell, the core and pyloric regions of the clot. Thus fat and casein leave the stomach in constant proportion.