The present investiagation was undertaken to examine possible change of the diurnal course of stomatal aperture in rice plants grown under different environmental conditions from the transplanting to the day of stomatal measurement. The environment was much different between the border and the interior of rice population of paddy fields, i.e., light intensity was weaker due to mutual shading, humidity higher and wind lower in the interior of paddy fields (Table 3). The stomatal aperture was both measured in rice plants grown in the border and interior of paddy fields and in those grown in the pots buried in the places mentioned above. Furthermore, the stomatal aperture of the rice plants which had grown in the interior was measured in the almost same condition as the border by reaping several rows in the south or west side of these rice plants by 10 m wide on the day just before the stomatal measurement. And also the stomatal aperture was measured in rice plants grown in the pots transported from the interior to the border on the day just before stomatal measurement. In this paper rice plants subjected to the two treatments mentioned above were represented by "Interior to Border", and rice plants grown and measured in the interior and border were represented by "Interior"and "Border", respectively. The stomatal aperture of rice plants which had grown under different environmental conditions was compared in the same border environment by measuring the diurnal course of stomatal aperture of "Interior to Border" and "Border". The stomatal aperture of rice plants which had grown under the same interior environmental conditions was compared in the different environment by measuring the stomatal aperture of "Interior" and "Interior to Border". The results obtained are as follows. On a cloudy day the diurnal course of stomatal aperture was practically the same in "Interior", "Interior to Border" and "Border" except when the stomatal aperture was smaller due to the severe decrease of radiation (Fig. 1. A.). On a fine day the stomatal aperture was not so different among three kinds of rice plants in early morning, but about 9 a.m. the stomata in "Interior" closed to a lesser extent than those in "Interior to Border" and "Border", due to weaker light intensity, higher humidity and lower wind in the interior of rice population, so that the stomatal aperture was larger in "Interior" compared with those in "Interior to Border" and "Border" at all growing stages (Fig. 1. A., Fig. 2-4). Furthermore, the extent of stomatal closure in "Interior to Border" was severer than that of "Border" in the afternoon on a fine day with intensive transpiration except for the two stages, i.e., the maximum tillering stage when rice plants did not grow so thick and the ripening stage when rice plants in the border were mature earlier than those in the interior (Fig. 2, 3). Attention should be paid to both facts that the stomatal aperture of rice plants grown in the pots buried in the border was larger in the afternoon on a fine day compared with those grown in the pots transported from the interior to the border, and the facts that the stomatal aperture in the pots transported from the border to the interior ("Border to Interior") was also larger compared with those in the pots buried in the interior (Fig. 4). The stomata of lower leaves closed to a more considerable extent in the afternoon than those of upper ones in "Interior to Border" compared with stomatal closure of upper and lower leaves in "Border" at the same hours (Fig. 5, 6. Table 5). [the rest omitted]