1008 REYSS-BRION (a) (h) Fig. 10— (a) Incorporation of tritiated thymidine in a control (after 2 hr). (b) Incorporation oftritiated thymidine in an irradiated explant (after 2 hr), there are no differences between controls and irradiated explants. In spite of the fall of the DNA synthesis, the RNA synthesis does not stop. Incorporation (lo to 100 pCi/culture medium) of D-L leucine (4-5-T). The uptake of leucine very much resembled that of uridine, but the cells are more heavily labeled. In some explants (controls or irradiated), however, the epidermis remained unlabeled in these conditions, suggesting periods of low protein synthesis (Fig. 11). Quantitative Determinations BEST AVAILABLE COPY The first quantitative determinations seem to confirm the observations, In the controls, thymidine is actively incorporated during the first 4 hr after administration. After 20 hr the total quantity of thymidine Incorporated decreases. The pattern of incorporation is the same in the frradiated explants as in the controls, but there is an inhibition of 20%, which ceases after about 20 hr. The incorporation of urldine and of leucine increases in the controls as in the irradiated. When we measured the same incorporation with higher doses of X-rays, we found that: incorporation of thymidine decreases strongly between 0 and 1600 R; incorporation of uridine increases till 1200 R, and decreases between 1200 and 1600 R; incorporation of leucine decreases very slightly when the X-ray doses are greater than 800 R.