The objective of this investigation was to measure the effects of captan on DNA, RNA and protein biosyntheses in limbs of developing chick embryo, in vivo. 2. Captan (12p.p.m.) was injected into the egg on day 4 of incubation and macromolecular syntheses were measured on days 8--14. 3. Total DNA content was unaffected by captan, but incorporation of [3H]thymidine was inhibited over the entire time range; the period of peak specific activity (day 11) was inhibited in treated samples to 67% of control. 4. Total RNA content was reduced in the earlier days but returned to normal by day 14, whereas incorporation of [3H]uridine was lowered throughout the period. The timing of peak specific activity for RNA synthesis was delayed by 2 days in treated eggs and was inhibited by 32% when peak days were compared. 5. Total protein concentration was slightly lowered by captan treatment in the mid-range of the days measured and the incorporation of [3H]valine was retarded in the early days; peak periods of synthesis were similar in magnitude but were shifted from days 9--10 in control to day 11 in treated embryos.
