The present study was carried out to determine, first, the cardiac beta-adrenergic receptor characteristics of broiler chick embryos exposed to two different CO2 levels during the last stage of embryonic development, and second, the prophylactic effect of beta(1)-adrenoceptor blocker on right ventricular hypertrophy in broiler chickens. High CO2 embryos showed significantly higher haematocrit values, higher partial pressure of CO2 levels and lower partial pressure of O-2 levels than those of normal CO2 embryos. Exposure of chick embryos to high CO2 levels reduced the binding capacity of myocardial beta-adrenergic receptors compared with those embryos incubated at the normal CO2 level. Atenolol-supplemented diet numerically reduced ascites incidence in broiler chickens (7%) compared with birds fed the control diet (15%). In conclusion, these data support the important role of the beta-adrenergic receptor in the cardiovascular system for cardiac output, and secondary to pulmonary hypertension syndrome.