The colonic epithelial cell line T84 has been shown to be a good model to investigate the regulation of Cl- secretion by the adenosine 3',5'-cyclic monophosphate (cAMP)-mediated second messenger cascade. Regulated exocytic insertion and endocytic retrieval of transport proteins, or proteins that regulate transport proteins, is one mechanism proposed to regulate plasma membrane solute permeabilities. The aims of our studies were to characterize endocytic processes in T84 cells and to investigate their regulation by known activators of Cl- secretion that are mediated by the cAMP second messenger cascade. Forskolin, an activator of adenylate cyclase, caused a marked inhibition of endocytic uptake of the fluid-phase marker horseradish peroxidase (HRP) and the adsorptive marker wheat germ agglutinin conjugated to HRP. Similar inhibition was obtained with vasoactive intestinal peptide, a secretagogue whose receptor is coupled to adenylate cyclase, and 8-(4- chlorophenylthio)adenosine 3',5'-cyclic monophosphate, a membrane-permeable cAMP analogue. 1,9-Dideoxy-forskolin, a forskolin analogue that fails to activate adenylate cyclase, was without effect on endocytosis. Our data show that the net rate of endocytosis, as measured by fluid-phase uptake, is decreased by a cAMP-mediated mechanism. Because the number of Cl- channels or associated regulatory proteins in the plasma membrane reflects a balance between their exocytic insertion and endocytic retrieval, we propose that the cAMP-mediated decrease in endocytosis could contribute to the concomitant increase in plasma membrane Cl- permeability.