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The synthesis of bacterial protein in the rumen of lactating cows fed on purified carbohydrates, with urea and ammonium salts as the sole sources of nitrogen, can be increased, through feed adaptation, to a level adequate not only for the maintenance of the cow but also for a relatively high milk production. The best annual milkyield per cow on the experimental feedhas, so far, been 4217 kilograms, calculated as standard milk (684 kilocalories per kilogram of milk). The composition of the test milk is similar to that of normal fat- and proteinrich milk. Fractionation of casein and serum proteins of test milk and normal milk by different methods demonstrated the similarity of the proteins of the two milks. The normal or higher concentrations of the water-solublevitamins in test milk show that the biosynthesis of these vitamins in the rumen is vigorous. The flavor of the two milks is very similar-proof that biosynthesis of the effective flavor compounds of milk occurs in the body of the cow. The only component of test milk whose composition, when small amounts of vegetable oil have been fed, has differed from that of normal milk is the fat. The studies of milk production on the experimental feed have opened up new possibilities for investigating the biosynthesis of different milk components. The studies are also of practical importance: Since the vigorous biosynthesis of proteins from simple nitrogen compounds in the rumen of test-feed-adapted cows has been demonstrated, there are greater possibilities for studying the replacement of protein by urea. In countries where there are plenty of forests, part of the feed of cows can be made up of certain wood products-for instance, hemicellulose and cellulose of low quality. The new findings may also be of value in the dry areas of the globe, where milk would be of vital significance for improving the nutrition of the population.