This study was undertaken to investigate the effects of indomethacin on 4-nitroquinoline 1-oxide (4NQO) induced palatal carcinoma of albino rats. Sixty albino rats about 100 gms of body weight, 6 weeks old-were used classifying as 1) six albino rats of normal group received no treatment, 2) six albino rats of control group treated with propane 1, 2-diol 3 times a week, 3) twenty four albino rats of experimental group I treated with 0.5% 4NQO in propane 1, 2-diol 3 times a week, 4) twenty four albino rats of experimental group II treated with 0.5% of 4NQO in propane 1, 2-diol 3 times a week and administrated 20${mu}g/ml$ indomethacin in drinking water ad lib. The animals of normal and control groups were sacrificed 7th, 11th, 15th, 19th, 23rd and 27th week, while those of experimental group I and II were sacrificed 7th, 9th, 11th, 13th, 15th, 17th, 19th, 21st, 23rd, 25th, 27th and 29th week after the experiment. The palatal mucosa was excised and examined grossly, light-microscopically and electron-microscopically. Following results were obtained. 1. In control group, there was no specific difference from normal tissue histopathologically. 2. In group I, hyperkeratosis mild acantosis and dyskeratosis in in 7th week, dysplasia in 11th week and severe acantosis in 19th week were observed, but squamous cell carcinoma not observed until 29th week on light-microscope. 3. In group II, hyperkeratosis, mild acantosis in 9th week, dyskeratosis and dysplasia in 21st week, severe acantosis in 27th week and squamous cell carcinoma in 29th week were observed on light-microscope. 4. In group I, widening of intercellular space in 7th week, increasing of desmosome, giant desmosome and tonofilament in cytoplasm in 9th week, severe widening of intercellular space, increasing of mitochondria and vascular degeneration in 11th week, irregular pattern of cell feature and nucleus and prominent nucleoli in 19th week, and continuity of basal lamina in 29th week were observed on electron-microscope. 5. In group II, mild widening of intercellular space in 9th week, increasing of mitochondria, vascular degeneration and tonofilament in cytoplasm in 13th week, increasing of desmosome and giant desmosome in 15th week, irregular pattern of cell surface and nucleus and prominent nucleoli, and in 21st week continuity of basal lamina were observed on electron-microscope which phenomenon occurred little later than group I. After 21st week, however, severe widening of intercellular space, vascular degeneration and continuity of basal lamina were observed as in group I.