Splanchnic and splenic erythrocyte volumes decrease during postural changes and exercise to help maintain central blood volume and cardiac output. The contribution of this compensatory mechanism to hemodynamic stability during dialysis has not been studied, however. In 8 ESRD patients, age 51.0 +/- 4.5 years old, we measured changes in the splanchnic/splenic erythrocyte volume during dialysis by tagging the patients' erythrocytes with technetium and following abdominal radioactivity over time. Splanchnic radioactivity decreased to 90.2 +/- 3.8% (mean +/- SEM) of the baseline value after 2 hr of accelerated fluid removal (3.7 +/- 0.4 liters) during dialysis (DUF), while it remained relatively unchanged after two hours of dialysis without fluid removal (DD) [106.5 +/- 2.3%, P (DUF vs. DD) = 0.03]. Splenic radioactivity decreased to 89.2 +/- 5.0% of the initial value during DUF versus 103 +/- 3.8% during DD, but the decrease was noted only during the last 30 minutes of DUF and did not attain statistical significance. Autonomic nervous system integrity was measured by the spontaneous variation of the R-R interval during deep respiration (E/I ratio) and by the Valsalva ratio. The mean E/I and Valsalva ratios in the eight patients were 1.13 +/- 0.03 (+/-SEM) and 1.42 +/- 0.1 respectively, suggesting reasonably adequate autonomic nervous system functioning. The results suggest that contraction of the splanchnic, and possibly the splenic, vascular beds occurs during fluid removal associated with hemodialysis. The resultant addition of erythrocytes to the circulation may help maintain central blood volume and cardiac output.