A reduced venous compliance (VC) and inadequate venoconstriction may impair hemodynamics during hemodialysis, the first by impairing plasma volume preservation and by inducing a steep fall in central venous pressure (CVP) during minor plasma volume loss, the second by inadequate mobilization of hemodynamically inactive blood volume. For the protocol A, the relation between VC, the fall in plasma volume and the decline in central venous pressure (CVP) was assessed in 12 hemodialysis (HD) patients, aged 40 to 74 years, during isolated ultrafiltration (UF). The patients were ultrafiltrated for one hour at an UF rate of 1 to 1.5 liter/hr. VC was measured by strain gauge plethysmography with direct i.v. pressure measurements. CVP was assessed directly via a subclavian catheter. PVP was measured using the serial hematocrit method. VC correlated inversely with the fall in plasma volume (r = -0.66; P less than 0.025) and with the fall in CVP (corrected for UF volume) (r = -0.62; P less than 0.025). In the protocol B, the constriction of veins and resistance vessels was assessed sequentially during isolated UF and during UF combined with bicarbonate HD (UF + HD) by measuring the change in venous tone (VT) and vascular resistance (FVR) of the forearm. Twelve HD patients were studied (age 30 to 64 years). VT and FVR were measured using strain gauge plethysmography. The UF rate was equal during isolated UF and UF + HD (1 liter/hr). In six patients, the measurements were started with isolated UF and in six patients with UF + HD.(ABSTRACT TRUNCATED AT 250 WORDS)