The effects of mica dust injected intratracheally and parenteral admini-stration of choline were studied in the lungs of albino rats. The intraperitoneal administration of choline (25 mg, 5 days/week) for 10 weeks caused dilation of lymphatic vessels along with adenomatoid changes in the lungs at 330 days. Mica alone (25 mg, intratracheal) provoked pulmonary fibrotic lesions with dilation of lymphatic vessels at 330 days. However, the combined action of mica dust and choline produced adenocarcinoma of bronchiolar epithelium at the termination of the experiment (339 days). Choline as an integral part of, cell membrane, its possible role in enhancing the reaction in the lungs exposed to mica dust is discussed.