We previously showed that acetylcholine (ACh) infused into the abdominal aorta of dogs at a rate of 127 micrograms ACh min-1 caused an increase in lumbar trunk lymph flow (L) of 35% while protein clearance into the lymph (LR) remained unchanged. These effects were accounted for by a 34% increase in reflection coefficient (sigma) and a 54% increase in permeability-surface area product (PS). Since arterial pressure decreased, it was possible that the decrease in arterial pressure was responsible for observed changes. The current study was undertaken to test this possibility. Seven female dogs were anesthetized and prepared in the same manner as the previous study except that control abdominal aortic pressure was reduced with an aortic balloon to a mean of 81 mmHg. As ACh was infused, the balloon pressure was released so that the mean pressure for all dogs rose to 96 mmHg. The findings indicated that ACh produced a 24% increase in L (P less than .004) while LR was unaffected. In a similar fashion to the results of the previous study, sigma increased 43% (P less than .0000) and PS rose 51% (P less than 008). These results clearly dissociate the effects of acetylcholine on permeability from any effects on arterial pressure and indicate a more direct effect of acetylcholine on the permeable segment. The results also suggest a general response of the capillary or postcapillary venule to vasodilation which restricts accession of protein into the interstitium during unloading of the vasculature by the process of edema formation.