The senior author has long been engaged to establish the chromosome map of rice plant by the usual F_2 analysis method. The results were rather fruitless notwithstanding the enormous F_2 combinations were analyzed. Recently the present authors tried to introduce another methods to promote this study, namely teisomic and translocation analysis using the lines which were obtained in the progenies of atomic bombed rice plants in Nagasaki. However, these materials were not desirable for trisomic analysis, the authors decided furthermore to propuce 12 kinds of primary trisomic lines from triploid plants artificially. This attempt was supported by the fact that 12 kinds of primary trisomic lines are theoretically well known as the haploid chromosome number of the rice plant is 12. In the first step, in order to obtain triploid rice plants, the authors attempted to crossbetween diploid and auto-tetraploid. During three years from 1959 to 1961, 9822 flowers were crossed reciprocally after usual hot water emasculation method. From these crossings, 77 normal fully developed grains and 947 wrinkled imperfect grains were obtained, although seed setting was better when the tetraploid plant was used as maternal plant than reciprocal crosses. Normal grains germinated well and grew to maturity, but only two of them, that is 0.022% of pollinated flowers, were true triploids and the others were maternal ones. Germination of imperfect grains were bad, representing 16% of 177 seeds sown, and no viable seedlings were obtained. Weight and germination rate of imperfect grains were also better in 4X×2X crosses than reciprocals. To make clear what is the cause of difficulty for getting triploid rice plants by crossing between diploid and tetraploid plants, embryological studies were carried on and the results were as follows. In the selfed seeds, both diploid and tetraploid, normal growth and development were observed and the 1st, 2nd and 3rd foliage leaves were differentiated on 5, 7 and 9 days after pollination respectively. Accumulation of polysaccharides in young primordia of embryonal organs proceeded quite normally and development of endosperm was also normal. In the hybrid seeds of 2X×4X, growth and differentiation were very poor and abnormal, that is, in the most advanced cases pro-embryo or sign of differentiation were only observed and no progress of differentiation were found. On the other sides endosperm developed pretty normally during 3 days after pollination, but soon afterwards it began to degenerate and entirely collapsed at the end. In the hybrid seeds of 4X×2X, development of embryo was better than that of 2X×4X, and the differentiation of 1st and 2nd foliage leaves were observed in the materials of 5 days and 7 or 9 days after pollination respectively. As compared with the selfed samples, differentiational feature was nearly equal to selfed 4X embryos and a little inferior to selfed 2X embryos and very malformed types were observed. In spite of such abnormal differentiation mentioned above, the growth was very vigorous and final size of the matured embryo was rather larger than the selfed ones. Development of hybrid embryo was thought to finish at 9 days after pollination. The development of endosperm was normal in earlier stage and exceeded the selfed one till 3 days after pollination but soon afterwards it rushed in degeneration process and most of the matured endosperm was collapsed. From the above experiment, the authors concluded that the difficulty of obtaining the triploid rice plants by crossing between diploid and auto-tetraploid plants was not due to the failure of fertilization but to the poor development of embryo and endosperm and unbalanced development of these organs. In addition, the authors discussed the introduction of the embryo-culture method and the improvement of crossing technique to facilitate the efficiency for getting triploid rice plants from the literature.