In ten uremic patients, who were not yet undergoing periodic hemodialysis and in whom we were creating a 1 cm, distal, side-to-side arteriovenous fistula of Brescia-Cimino type for hemodialysis, the acute hemodynamic changes of the systemic and pulmonary circulations were studied immediately after the opening of the fistula. An increase in cardiac output (Q) was observed in one patient, the other patients showing either no change or a slight reduction. In the four patients in whom the Q decreased there was a significant reduction of total blood volume (TBV) and stroke volume (SV) and an increase in systemic vascular resistance (SVR) (p less than 0.05, p less than 0.05, p less than 0.01 respectively). At the level of pulmonary circulation, in these patients a decrease in pulmonary blood volume (PBV) (mean = 20%) and a significant increase in pulmonary vascular resistance (PVR) were also observed. In five patients who had been on chronic hemodialysis and who presented the clinical picture of cardiac failure, the acute hemodynamic changes following temporary closure of the fistula (by a sphygmomanometer) were studied: a significant decrease (p less than 0.05) in Q, TBV and SV was observed. The difference between the two values of Q (i.e. fistula open and closed) was considered to indicate the magnitude of the flow across the fistula.