1. In the studies reported here, broiler lines divergently selected for susceptibility to ascites under low temperature conditions were tested for their sensitivity to 3,3',5-triiodothyronine (T3) with respect to growth rate, rate of mortality, plasma concentrations of T3, right ventricular hypertrophy (RVH) and incidence of ascites. 2. Mean body weight of the ascites-susceptible line (BC-line) was higher than that of the ascites-resistant line (A-line). Adding 0.5 mg T3/kg of the diet depressed growth rate to the same extent in both lines. The effect of T3 on growth was more pronounced for males than for females. 3. T3-supplementation increased the relative weight of the heart and the incidence of RVH to the same extent in both lines. More of the T3-treated BC-line chickens had fluid accumulation in the abdominal cavity than the T3-treated A-line chickens. 4. Dietary T3-treatment depressed the plasma concentration of growth hormone (GH) profoundly and insulin-like growth factor (IGF-I) slightly but to the same extent in both lines. The coefficient of variation of GH concentrations indicate that T3 treatment mainly decreased GH-pulsatility in young growing broilers. 5. Higher doses of dietary T3 (1 and 2 mg/kg) increased mortality in a dose-dependent manner. With 2 mg T3/kg, mortality in the BC-line was almost double that in the A-line. 6. These studies indicate that the development of ascites could be linked with thyroid function. Moreover, dietary T3 supplementation could be used to help identify ascites-inducing factors or genetic lines with differential sensitivity for ascites.