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Department of Agriculture has studied the effects of close inbreeding in the guinea pig. Twenty-four families were started and continued by brother to sister matings. Five of these families were maintained until 1933, but since then only families 2, 13, and 35, and certain crosscs betwcen thcse, remain. I n 1911 a control stock B was set up f o r comparison with the inbred lines. This control stock has been maintained by matings between animals which wcre no morc closely rclated than third cousins. Both the inbred and control animals have been maintained under identical conditions at the experimentaI farm of the U. S. Departmcnt of Agriculture at Beltsville, Maryland. During this long period of obscrvation differences in growth rate, fertility, mortality, color markings and other characteristics among various inbred lines, and between them and thc control stock have been noted, with thc differences in growth being the most marked. In the inbred families there has been a markcd reduction in fertility and a considerable reduction in growth rate. Prenatal and postnatal mortality have increased mrkcdly. That these declines in vigor wcre due in Considerable dcgrce to environmental condi-'Those studies were aided by a grant made t o Vwderbilt University by the Division of Medical Scienees of the Rockefcllcr Foundation.