We have studied the chicken's pineal gland by light and electron microscopy at 3 and 4 weeks of age. The results indicated that lymphocytes, plasma cells, secretory cells, basophil and eosinophil granulocytes enter the parenchyma and a new histologically defined tissue is formed which we call lymphopineal tissue. Inside the pineal parenchyma, the number of thymidine labeled cells is almost three times higher than in the interstitium suggesting that the pineal's products might be blastogenic for the cells and/or exert an influence on post-thymic T-cell differentiation.