In male dd-mice aged at 20, 35, 70, and 150 days, the pulmonary lymphatic tissues were histologically studied. In adults, the lymphatic tissues were divided into two forms, hilar and subpleural. The hilar lymphatic tissue occurred as nodular accumulations of small lymphocytes around the bronchial branches at the hilar region. The nodules, measuring 100 to 120 micrometers in diameter and about 150 micrometers in length, were generally found around small veins or venules. No hilar lymphatic tissue was found until 20 days of age. Small nodules increased in number with advancing age. In adults older than and 70 days of age, the number of the nodules was 5 to 7 in the left lung and 7 to 12 in the right lung. The subpleural lymphatic tissue was found beneath the pleura in the peripheral region of the lung. It also occurred as small nodules around small veins. The nodules consisted mainly of small lymphocytes. In addition, they contained a small number of younger lymphocytes, plasma cells, and large mononuclear cells. The subpleural lymphatic tissue was not found in all the adults thus far examined. It was considered to occur as an immune response.