The submandibular gland transports fluid at a high rate through the interstitial space during salivation, but the exact level of all forces governing the transcapillary fluid transport has not been established. In this study we aimed at measuring the relationship between interstitial fluid volume (V_i) and interstitial fluid pressure (P_if) in salivary glands during periods of active secretion and after systemically induced passive changes in gland hydration. We tested if interstitial fluid could be isolated by tissue centrifugation to be able to measure interstitial fluid colloid osmotic pressure (COP_i). During control conditions V_i averaged 0.23 ± 0.014 ml/g wet weight, with a corresponding mean Pif measured with micropipettes of 3.0 ± 1.3 mmHg. After induction of secretion by pilocarpine, P_if dropped by 3.8 ± 1.5 mmHg whereas V_i was unchanged. During dehydration and overhydration up to 20% increase of Vi above control, a linear relationship between the volume and pressure was found, resulting in a compliance (V_i/P_if) of 0.012 ml/g wet wt/mm Hg. Interstitial fluid was isolated and COP_i averaged 10.4 ± 1.2 mmHg, 64% of the corresponding level in plasma. We conclude that P_if drops during secretion and thereby increases the net transcapillary pressure gradient, a situation that favour fluid filtration and increase the amount of fluid available for secretion. The reduction in P_if is most likely induced by contraction of myoepithelial cells and suggests an active and new role for these cells in salivary secretion. The relatively low interstitial compliance of the organ will enhance the effect of the myoepithelial cells upon P_if during conditions of reduced V_i.