Quarts powder was injected intratracheally into rabbits to induce experimental pneumoconiosis. Simultaneous intravenous injections of chloroquine diphosphate or gastric intubation of tetrahydroxyquinone (THQ) were given. This was done with the expectation of preventing or delaying pulmonary fibrosis. As a result, though silicotic changes similar to those of the control group were found in the THQ-treated group, no such changes occured in chloroquine diphosphate-treated group. Silicotic changes in the THQ-treated and control groups were as follows: The lungs showed proliferation of septal macrophages, thickening of the alveolar wall, scattered quartz powder, and foreign body giant cells. These silicotic changes were seldom seen in chloroquine diphosphate-treated lungs. It appeared that chloroquine diphosphate was inhibitory to early silicotic changes and acted to prevent the dvelopment of silicosis.