1. The relationship between the length of incubation, hatching, hypoxic condition, thyroid hormones and the occurrence of ascites were studied in embryos of 2 broiler lines differing in susceptibility to the ascites syndrome. 2. Both the time of external pipping (ep) and hatching of embryos from the ascites-resistant (AR) broiler line was earlier compared to the ascites-sensitive line (AS). The interval between internal pipping (ip) and ep was the same between the lines, but the interval between ep and hatching was shorter in the resistant line. 3. The T3 and T4 concentrations in plasma of the AS line were lower compared to the AR line. 4. Analyses of partial pressures of oxygen (pO2) and carbon dioxide (pCO2) in the air cell of the egg revealed that at day 18 the AS embryos had lower pO2 and higher pCO2 concentrations compared to the AR embryos. 5. The delay in hatching of the AS embryos might induce a more pronounced and/or extended hypoxic environment, thereby creating an environment that evokes ascites.