SummaryThe number of lymphocytes discharged from the mesenteric lymphatic duct was measured over several days of continuous lymph drainage in adult guinea pigs which had been thymectomised at birth or as young adults. The reduced output of lymphocytes during the first few days of lymph drainage from neonatally thymectomised guinea pigs indicated that removal of the thymus at birth produced a deficiency in the mobilizable lymphocyte pool. Adult thymectomy led to a decrease in the moblizable lymphocyte pool which did not become apparent until 35 days after removal of the thymus. This indicated that the production of lymphocytes by the thymus is important in the guinea pig for maintaining adequate numbers of recirculating lymphocytes during adult life.The concentration of lymphocytes was reduced in the peripheral blood of guinea pigs which had been thymectomised at birth. In female guinea pigs this lymphopenia was accompanied by an increase in the number of lymphocytes hearing Kurloff bodies. The concentration of lymphocytes in bone marrow was not significantly altered following neonatal thymectomy.