Two isozymes of carbonic anhydrase (EC 4.2.1.1.) were compared in the anterior pituitary gland of non-tumor-bearing rats and in the hormone-secreting pituitary tumors. In contrast to the pituitary gland, which contained 60 to 70% of the total carbonic anhydrase in the particulate subcellular fraction, three hormone-secreting pituitary tumors were devoid of the particulate (Triton X-100-solubilized)enzyme activity. Another pituitary tumor, 7315a, contained particulate carbonic anhydrase, but the activity was only 45% of the activity of normal pituitary gland. During the development of the rat brain, the particulate (Triton X-100-solubilized) carbonic anhydrase activity was undetectable in prepartions up to 21 days of age (body weight, 47 g). After that age, the carbonic anhydrase activity in the particulate fraction increased rapidly and reached the adult level at 37 days (body weight, 120 g), while the activity in the soluble fraction increased gradually after birth and then reached a plateau at 30 days (body weight, 81 g). These data show that the isozyme pattern of carbonic anhydrase in pituitary tumor tissue resembles the pattern in fetal cells more than the pattern in adult tissue.