To evaluate the normal haemodynamic and respiratory responses to blood-membrane contact sham-dialysis, i.e. with blood flowing through a dialyser but without dialysate and ultrafiltration, was performed on healthy young men during 150 minutes. Heart rate, cardiac output, arterial blood pressure and pulmonary arterial blood pressure (continuously recorded during the initial 30 minutes) did not change significantly. The white blood cell count fell markedly to a minimum after 20 minutes of blood-membrane contact, then returning to above the baseline values, but PaO2 did not change significantly.