The basal rate of amylase secretion from rat pancreatic lobules was found to diminish with time. This decline, which is not due to depletion of amylase stores or loss of tissue integrity, has been attributed to the build-up of amylase in the extracellular space [Ho and Rothman, Am. J. Physiol. 245 (Cell Physiol. 14): C21-C27, 1983]. This accumulation of amylase would result in a decrease in the intracellular-extracellular amylase gradient, which, according to the so-called equilibrium hypothesis, is responsible for net amylase secretion under basal conditions. Using rat pancreatic lobules and acini, we have tested the validity of this hypothesis by adding to the incubation medium at the onset of incubation either collected secretory products or purified amylase. Based on the equilibrium hypothesis, one would have predicted that these additions would also reduce the concentration gradient favoring net secretion and would result in an apparent reduction in the initial rate of basal in vitro digestive enzyme secretion. The results obtained from these studies, however, were not in accord with those predictions, since these additions did not diminish the initial rate of basal amylase secretion. Our observations therefore indicate that the decrease in the rate of basal amylase secretion from rat pancreas with time is not due to the loss of a concentration gradient favoring efflux. These results argue against the validity of the equilibrium hypothesis.