Ascites is a metabolic disorder that accounts for over 25% of overall mortality in the broiler industry. This disorder is manifested between wk 5 and 6 posthatch, but there are previous indications that predisposition may be identified during embryonic development. In this current study, we determined embryonic physiological and metabolic parameters that maybe associated with ascites predisposition. For this purpose, we used broiler eggs from 3 lines that differed in ascites. sensitivity. These included an ascites-sensitive dam line (DAS), an ascites-resistant dam line (DAR), and an ascites-sensitive sire line (SASL). Eggs were incubated for 21 d under standard conditions. The following parameters were measured during incubation: egg weights at setting, egg weight losses at 18 d, embryo body weights and embryo heart weights throughout development, air cell partial gas pressures (pCO(2) and pO(2)) levels at d 18 and at internal pipping (IP); plasma triiodothyronine, thyroxine, and corticosterone levels at d 18, 111, and hatch; heat production from d 17 until hatch, hematocrit Values at hatch, and posthatch growth rate to 7 d along with hematocrit values. The data obtained revealed that,election for ascites sensitivity or rapid growth rate had no consistent influence on some of these parameters such that they could be wholly associated with ascites sensitivity for predictive purposes. Whereas differences in embryonic developmental patterns were apparent throughout embryonic development, these differences in physiological and metabolic parameters may be due partly to genetic differences unrelated to ascites sensitivity.