In this investigation we believe that we have learned something suggestive concerning the thymus gland, and, what is equally important perhaps, we have evolved a novel procedure for the study of this and possibly other endocrine organs. Following the continuous administration of thymus extract to successive generations of rats, marked acceleration in the rate of growth and development has been observed during the early life of the offspring, particularly of the third and later generations. Thus the rate of development encountered in the fifth generation of young rats born of four generations of forbears treated with a thymus extract is almost beyond belief. METHODS OF EXPERIMENTAL PROCEDURE A colony of 12 white rats was obtained from the Wistar Institute of June 16, 1933. These animals were divided into test and control groups. Up to the time of writing (April 16, 1934) the test animals have received by intraperitoneal injection 1