We evaluated the effect of IV crystalloid administration on hematocrit in 28 healthy, nonbleeding volunteers. Normal saline boluses of 10, 20, and 30 mL/kg were delivered at a mean rate of 115 +/- 4 mL/min, followed by a continuous infusion of either 1 or 5 mL/kg/hr. Hematocrit values determined immediately after bolus infusion declined from baseline a mean +/- SEM of 4.5 +/- 0.6 (P less than .001), 6.1 +/- 0.4 (P less than .001) and 6.3 +/- 0.6 points (P less than .001) in the 10, 20, and 30 mL/kg groups, respectively. Twenty minutes into the maintenance infusion, hematocrits had risen 1.5 +/- 0.8 (P = .03), 2.4 +/- 0.4 (P = .004) and 2.3 +/- 0.7 points (P = .005) when compared with post-bolus hematocrits in the 10, 20, and 30 mL/kg groups, respectively. These data show that in healthy, nonbleeding subjects, crystalloid infusion may cause significant changes in hematocrit. Calculations reveal that approximately 60% of infused saline, when delivered as a bolus, will diffuse from the intravascular space within 20 minutes of administration.