Plasma volume (PV) and red cell volume (RCV) transient changes due to rapid (1 min) infusion of 12 ml/kg of 0.833 M NaHCO3 solution were measured in dogs. The PV transient was measured using the "indirect" method with 125I-albumin dilution and measurement of RCV with 51Cr tagged red cell dilution, large vessel hematocrit, and F cell ratio. Both methods gave results of an 80 percent peak overshoot of PV immediately following the infusion and then a fall over the next 120 min to a new steady state. The RCV fell 5 percent immediately after infusion and then returned in 20 min to slightly lower than control. The PV transient indicates an initial rapid absorption of water due to an increase in plasma osmolality; then a balance of forces at the peak PV, possibly through increased capillary hydrostatic pressure, decreased tissue pressure, and decreased capillary osmotic pressure due to dilution of plasma proteins. This was followed by a gradual filtration of water out of the vascular system. 27 pp. Ref.