The present study examined the influence of humidity on the photosynthesis in rice plants grown at a high or low humidity. 1) Plants were grown for 2-3 weeks at a high-intensity light with high or low humidity. Photosynthetic rate measured under high-intensity light was higher for plants grown in low humidity than those grown in high humidity. However, when it was determined under low-intensity light, the rate of the latter was higher than that of the former (Fig. 2). 2) The specific leaf weight was greater in the plant grown in low humidity than in that grown in high humidity. It had possitive correlation with photosynthetic rate under high-intensity light, but negative correlation, though it is not significant, under low-intensity light (Fig. 3, 4). 3) The photosynthetic rate was measured under high- or low-intensity light at various air humidities. Irrespective of light intensity, the more humid the atmosphere during photosynthesis measurement, the higher the photosynthetic rate of leaves of the plants grown under both low and high humidity (Fig. 5). Furthermore, the photosynthetic rate decreased, when the air humidity was lowered during measurement of photosynthesis, while it increased when the humidity was increased during measurement (Fig. 6). 4) Stomatal resistance decreased with the increase in humidity, suggesting that stomatal aperture increased with the rise of humidity (Table 1). These results suggested that air humidity which influences stomatal openining is a direct factor affecting photosynthesis and air humidity under which plants were grown for 2-3 weeks before photosynthesis measurement also has some effect on leaf morphology, which results in dfferences in photosynthetic rate.