The T-wave characteristics, heart rate, stroke volume, cardiac output and blood pressure were examined in acidosis-prone Pietrain and acidosis-resistant Gloucester Old Spot pigs, before and after autonomic blockade with propranolol, atropine and phentolamine. Assessments were made at normal arterial blood pH and pCO2, and at either the same age or live weight in the two breeds. T-wave amplitude was influenced by alpha-adrenergic activity in the absence of beta-adrenergic and parasympathetic receptor activation. The initial cardiac output was 25% greater in the Pietrains, and 75% of this breed difference was due to autonomic factors. Myocardial chronotropic responsiveness to noradrenaline was similar in the two breeds. Initial blood pressure was greater in the Gloucester Old Spots because of a non-autonomic factor, and vasopressor responsiveness to noradrenaline was greater in the Gloucester Old Spots. Although acidosis-prone pigs may be afforded some degree of protection from a sympathetic crisis by having a low vasopressor responsiveness to adrenoreceptor stimulation, enhanced catecholamine output is the likely cause of their stress-induced myocardial lesions. A high catecholamine output may also account for the reduced body fatness in this type of pig.