['This study was designed to see if humorally mediated negative inotropism contributes to the cardiac output (CO) depression seen during positive end-expiratory pressure (PEEP) ventilation.', '8 pairs of piglets were used and a combined blood circulation was established between the two animals in each pair. One animal was ventilated with a PEEP of 15 cmH20 (donor) and the other was ventilated without PEEP (recipient).', 'CO and stroke volume deteriorated in the donors by 32% and 44%, respectively, while no change was seen in the recipients.'] Does pEEP ventilation cause humorally mediated cardiac output depression in pigs?