Objective: We have recently reported that the initial distribution volume of glucose (IDVG) reliably measures the central extracellular fluid (ECF) volume in the presence of fluid gain or loss. However, it is not clear if IDVG consistently reflects central-ECF volume when redistribution of fluid occurs in the absence of fluid gain or loss. This study was designed to investigate changes in fluid volumes during phentolamine infusion in dogs. Design: Prospective animal study. Setting: Institutional animal research laboratory. Subjects: Fourteen anesthetized and ventilated mongrel dogs. Interventions: Anesthetized animals were mechanically ventilated and received infusions of normal saline (n = 7) or phentolamine (10 μg kg min) (n = 7). Plasma volume was estimated using the indocyanine green (ICG) dilution method (PV-ICG) and IDVG was calculated using a one-compartment model by simultaneous administration of ICG 0.5 mg/kg and glucose 100 mg/kg before, during, and after infusion of either drug. Measurements and results: PV-ICG during infusion was not different between groups. However, IDVG significantly decreased (P < 0.05) following phentolamine infusion when compared with normal saline infusion. Conclusion: Our results suggest that IDVG rather than PV-ICG consistently measures central extracellular fluid volume, even when redistribution of fluid occurs.