Abstract: Ethrel stimulates the translocation of assimilates, both preserved and those formed after treatment, into cotton bolls, particularly, into fibres. It seems that the improvement of quality and fullness of fibre by ethrel was achieved by altering the translocation and distribution of photosynthetic products and increasing their economic coefficient. But the allocation of photosynthetic products in the cotton seeds in somewhat decreased, which subsequently has certain influence on seed quality. In the shell, the allocation of photosynthetic products has little difference between the treated and the control. Within a week after treatment, the photosynthetic efficiency shows no remarkable change, but decreases significantly thereafter. Thus, if ethrel is applied too early, it may affect the fullness of maturing cotton bolls due to precedable senescence of cotton leaves. The respiratory intensity of cotton leaves is rapidly increased by ethrel but it tends to be lower than control 6 days later.