Effect of distension pressure (d.p.) on the rate of lymph flow, serosal transudation and fluid transport was studied on an in vivo upper jejunal preparation. In the non-absorptive state with isotonic Na2SO4 solution as the luminal fluid, neither lymph flow nor transudation was increased with increasing d.p. from 0 to 3, 10, 20, 40 or 70 mmHg. Fluid absorption rate (Jv) increased by more than 100% when d.p. was increased from 0 to 3 mmHg whether glucose was present or absent in the luminal fluid (Krebs-Ringer solution). In the presence of glucose, there was no difference in Jv in the range of d.p. between 3 and 70 mmHg. In the absence of glucose Jv decreased when d.p. was above 10 mmHg. Whether with or without glucose, lymph flow did not change between d.p. of 0 and 10 mmHg but decreased at higher pressures. Whether with or without glucose, transudation began to appear at d.p. of 10 mmHg, increased with increasing d.p. up to 70 mmHg, but was much less in the absence of glucose. Protein concentration in the lymph (1.5-2.4% (w/v] and transudation (1.5-2.1% (w/v] was practically the same, suggesting that the latter could have originated from the lymph which leaked out of the serosal membrane. Blood flow in the subserosal capillaries and venules was remarkably reduced when d.p. was above 10 mmHg. In both jejunum and ileum, d.p. never caused fluid secretion during distension but resulted in transient fluid secretion as well as transudation following distension, apparently due to increased capillary filtration as a result of increased capillary permeability by ischaemia (during distension). From the effect of d.p. on the increase of transudation, presumably lymph, it is concluded that luminal hydrostatic pressure may play a role in fluid transport via the lymphatic system.