Summary The dog, unlike many species of laboratory animals, is thought to be immunologically mature at birth. Some investigators feel that the thymus gland has performed its major function in utero, and that neonatal extirpation of the gland fails to alter the immunobiology of the puppy. The results in the animals in this experiment parallel remarkably those recognized in the small laboratory rodents which themselves have undergone thymectomy. Several weeks after removal of the gland, the puppy becomes wasted, hunched and cachectic. The number of circulating peripheral lymphocytes and gamma globulins diminishes markedly within two weeks after operation when compared to similar animals with sham thymectomy. At postmortem examination, the lymphoid tissue of the dogs with thymectomy showed decrease in numbers of lymphocytes and germinal centers. Because of the profound physical, hematologic and histologic alterations found in this study, we feel that the lymphoid system of the dog is less mature at birth than previously thought. These changes warrant further investigation in this species.