The isolated perfused submandibular salivary gland of the rabbit has been used in order to make estimates of the filtration coefficient (Kf) and reflection coefficient (sigma d) of the capillary wall to albumin. 2. An isogravimetric preparation was used and in paired experiments the value for Kf obtained in glands perfused with albumin-Krebs solution, 0.96 +/- 0.086 (mean +/- S.E. of mean) ml min-1 mmHg-1 100 g-1, was not significantly different from that in blood-perfused glands, 0.90 +/- 0.15. 3. On analysing the data for reflection coefficient, it was concluded that the above values underestimated Kf by about 30%; using corrected values for Kf, osmotic reflection coefficients were determined from the weight changes following a sudden change in the oncotic pressure of the perfusate. The value for sigma d to albumin lay between 0.79 and 1.0, the lower value being obtained after the Kf correction. 4. The high hydraulic conductivity, combined with sieving properties comparable to those in continuous capillaries, is consistent with other data on fenestrated capillaries. 5. Finally, it was observed that, while the Kf value calculated from the initial flux rate was similar whether measured during fluid efflux from or influx into the microvasculature, on returning to the initial conditions after raising osmotic pressure, efflux was now more rapid than influx. This phenomenon is discussed in relation to readjustment of Starling forces and the possible existence of an asymmetric double membrane in the capillary, interstitium system and cells of the salivary gland.
