Mild diabetes (blood glucose ranging from 130 to 200 mg/100 ml) was induced in female rats by streptozotocin administration prior to mating. Fetuses obtained from these diabetic mothers on 20.5, 19.5 and 18.5 days of gestation were compared with corresponding fetuses from control mothers. Macrosomia was defined as a fetus whose body weight exceeded the mean + 2.5 SD of fetal weight obtained from the control fetal population. Similarly, the rate of substrate influx was considered abnormally high when an individual value (cpm/g tissue of radioactive tracer) exceeded mean + 2.5 SD of control value. The frequencies of macrosomia in control groups and in diabetic groups were 1/98 and 15/110 respectively, on 20.5 day of gestation, 1/155 and 14/ 148 on 19.5 day, and 0/99 and 0/98 on 18.5 day. The rates of influx of thymidine, leucine, and alphaaminoisobutyric acid from maternal side to the macrosomic fetuses were increased as much as 20 times the corresponding rates in control fetuses. Despite the lack of macrosomia as defined here in the fetal population on 18.5 day of gestation, a significantly larger proportion of litters from diabetic mothers (5 litters/5 litters) contained one or more fetuses showing abnormally high rate of thymidine influx than the litters from control mothers (0 litter/5 litters). These results suggest that only a selected few fetuses in a litter from a diabetic mother become macrosomic and a marked stimulation in the rate of metabolite influx into fetuses precedes the manifestation of macrosomia.