The development of osmotic pressure, following the addition of bovine serum albumin (BSA) to the perfusate, was measured in an isogravimetric cat hindlimb fully dilated with papaverine. The changes in colloid osmotic pressure were followed by recording the changes in venous pressure (PVI) necessary to maintain constant weight. Capillary filtration coefficient (CFC) and vascular compliances were calculated from weight changes following steps in venous pressure. Venous BSA levels reached arterial within 1 min, and PVI reached a new steady state 13.1 +/- 0.9 (SE) min (n = 9 in 6 limbs) after adding BSA. This apparent delay in the development of the osmotic pressure agrees with earlier reports. However, when the BSA was added under constant venous pressure conditions, the fluid reabsorption reached its maximum rate within 1.5 min. The data suggest that the BSA is osmotically fully effective after the capillary washin. The slowness of the rise of PVI in the isogravimetric state is probably caused by the time necessary to absorb sufficient fluid from the interstitium to raise the capillary pressure to a level that balances the increased oncotic pressure. A mathematical model supports this explanation.