1. Jejunal compliance (delta V/delta P) was calculated from the intraluminal pressures measured in anesthetized dogs in an in situ upper jejunal pouch (40-50-ml capacity) with intraluminal volumes of 10, 20, 30, 40 and 50 ml of isotonic saline. 2. Measurements were made in the same animal during and after acute and sequential alterations of the extracellular fluid (ECF) volume obtained by: a) acute intravenous (iv) infusion of isotonic saline, b) acute hemorrhage, and c) iv reinfusion of isotonic saline. 3. Expansion of the ECF volume caused a significant, reversible downward shift of the compliance curve, i.e., the jejunal pouch became less receptive to liquid distension. After saline infusion was discontinued, compliance gradually returned to control levels. 4. Acute loss of a substantial volume of blood after ECF expansion gradually shifted the compliance curve upwards to levels significantly different from control, indicating that retraction of the ECF volume made the jejunal pouch more receptive to liquid distension. 5. Reinfusion of bled animals with saline rather than autologous blood also induced a significant decrease in jejunal compliance to below control levels. 6. The jejunal pouch behaved as a suitable preparation for monitoring in vivo modifications of compliance induced by acute changes in ECF volume, especially when it was nearly "half-full" (i.e., filled with 20 ml), suggesting a critical relationship between the volume capacity of the pouch and its fluid content. 7. These results suggest that the modulation of the jejunal portion of small intestine compliance is involved in the processes that balance the ECF volume during acute life-threatening situations such as accidental hyperhydration or hemorrhage.