Background: Milk formulas for artificially reared (AR) rat pups are mostly based on complex cow's milk products, prepared by laborious and time-consuming processes. The aim of this study was to develop a simplified procedure for preparing a refined formula and to examine its influences on gut and brain development. Methods: The formula comprised a combination of purified cow's casein and whey proteins, five kinds of edible oil, minerals, and vitamins. Detailed analyses showed that the composition of macro- and micro-nutrients, osmolarity, and pH of the new formula closely resembled those of rat's milli. Rat pups, each with an intragastric cannula implanted at age 5 days, were artificially reared for the following 10-15 days. Results: The body weight gain of AR pups matched that of mother-reared (MR) pups. Histoplanimetrical analyses showed that the small intestine in AR pups was more developed In relation to area of a transverse section, number and length of villi, and thickness of tunica muscularis than that of MR pups. Fat components in the formula influenced the fatty acid composition and the choleslerol-to-phospholipid ratio in the sm all intestinal microvillus membrane (MVM) of AR pups, but not the MVM fluidity. Brain weight was not significantly different between the two groups at age 15-20 days. Conclusion: This formula is useful for artificial rearing of rats and for identifying dietary components contributing to metabolic adaptation during the suckling period. (C) 1997 Lippincott-Raven Publishers.