It is generally advocated to use saline or albumin infusions during symptomatic hypotension during dialysis. However, because of their side effects and/or costs, they are of limited use. Hydroxyethylstarch (HES), a synthetic colloid with a long-standing volume effect, is used in the management of hypovolemia. In this study, the efficacy of three fluids (isotonic saline [0.9%], albumin [20%], and HES [10%]) was assessed during three treatment sessions with combined ultrafiltration and hemodialysis, which differed in the type of fluid given intravenously. Changes in relative blood volume (BV), systolic BP (SBP), and vascular reactivity (venous tone [VT]) were compared. An intravenous infusion of 100 ml of fluid was given when the decrease in BV versus baseline was more than 10% as measured by a continuous optical reflection method. The ultrafiltration was continued. BV decreased significantly versus baseline independent of the intravenous fluid administration in all three treatment sessions. However, when we compared BV values at the end of the dialysis session with those at the time of infusion, BV continued to decrease significantly with saline (change in BV -4.56 +/- 2.75%; P < 0.05) and albumin (change in BV -2.13 +/- 2.51%; P < 0.05), but not with HES (change in BV -0.15 +/- 2.17%; NS). Between albumin and HES there were no significant differences in changes in BV (NS), whereas between HES and saline (P < 0.05) and between albumin and saline (P < 0.05) the differences in BV changes were significant. SBP remained unchanged within each session. Although SBP tended to decrease more with saline compared to albumin and HES, the difference was not significant. The higher decrease in BV and SBP with saline was counterbalanced by a significantly higher increase in VT, while VT remained unchanged in the other two sessions. It is concluded that HES is a promising fluid in preserving blood volume, comparable to albumin, but superior to saline.