Male progeny from two commercial sire strains, were fed on a high energy or low energy diet and kept at an environmental temperature of 23 C or 13 degrees C after day 22 to day 57. In the cold 42.4% of strain A and 29.6% of strain B died from right ventricular failure (RVF) and ascites secondary to pulmonary hypertension (PH) prior to day 57. Of the birds surviving to day 57, 9.4% (strain A) and 6.4% (strain B) had RVF and ascites. In the warm, 17.3% (A) and 7.4% (B) died from RVF and ascites before day 57 and 4.6% and 3.2%, respectively had ascites at day 57. Chickens on the high energy diet were significantly heavier at day 56 than those on the low energy diet, particularly those kept in the cold. However, chickens on the latter diet had better weight gain between days 42 and 56, again more marked in cold conditions. The higher metabolic rate elicited by cold would increase oxygen consumption and blood flow in these broilers which, because of their rapid growth, already had a high metabolic rate. The cause of PH was probably insufficient pulmonary vascular capillary capacity for the blood flow necessary to meet their metabolic oxygen requirement.