A study of lymphoid organ sections and imprints of normal hamsters from fetal to adult state revealed a sequential pattern of organ structure and growth. The thymus developed an adult structure by 1 week, maximum size at 6 weeks. The mesenteric lymph node had an adult structure by 4 weeks, the spleen by 6 weeks, with an accelerated growth of both organs between 6 to 8 weeks. The growth and development of the peripheral lymphoid organs appears to be under thymic control until the attainment of a "critical peripheral lymphoid mass." This appears capable of functioning independently of thymic control. Thymectomy is effective in producing lymphoid atrophy with resultant immunosuppression and wasting disease only if performed prior to the attainment of the "critical peripheral lymphoid mass."