1. The effect of propylthiouracil (PTU)-induced hypothyroidism on carbohydrate and lipid metabolism was studied in the chick embryo. 2. A single dose of PTU (250 micrograms/embryo) was administered on day 11 and embryos sacrificed on day 20 of incubation. 3. Thyroid glands were significantly enlarged (6 fold) by PTU administration. 4. Increased thyroid weight was associated with growth retardation and decreased plasma thyroxine levels. 5. Plasma glucose level was lower and phospholipids were significantly higher in the hypothyroid embryo. 6. Liver lipid concentrations in the control and hypothyroid embryos were not different but were significantly higher in both groups when compared to previously reported values in the young chick. 7. In contrast to PTU treatment after hatching, liver glycogen levels were not increased in the hypothyroid chick embryo. This was attributed to the high lipid nutrient condition of the chick embryo since a high lipid diet in the young chick decreased hepatic glycogen accumulation significantly.