A three factorial experiment was conducted to test genetic susceptibility for ascites interacting with experimental factors as hyperthyroidism and reduction of egg shell conductance during incubation. 120 eggs from 2 genetic stocks (ascites-resistant (R) and ascites-sensitive (S) populations) were taped below the air cell day 1 of incubation (T) while the remaining 180 eggs from each stock were incubated untaped (NT). At the day of hatching chickens from each genetic line and from the taped and untaped stocks were divided into 2 groups: one group receiving a T-3-supplemented diet (1 ppm) (T-3) and the other receiving a control diet (C). Weekly 25 chickens from each group were weighed until 7 weeks of age and from week 2 on blood samples were collected from 10 randomly selected chickens from each group for measurement of hematocrit, plasma lactate, pyruvate and T-3. Ascites mortality was daily recorded throughout the experiment. At the end of the experiment 10 chickens per group were killed and examined for right ventricle hypertrophy (RV/TV ratio) and other symptoms of ascites.