We measured stimulant-induced changes of exocytosis that are associated with increases in Cl secretion (i.e., short circuit current, Isc, in gA/cm2) and apical (ap) Cl permeability (Pa) and basolateral (bl) K permeability (PK) (both in cm/s) in T84 monolayers. Pc0 and PK were measured by permeabilizing the bl or ap membrane with nystatin. Pc0 was also measured with a fluorescent dye 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ). A noninvasive and sensitive method (release of 3`So4-labeled glycosaminoglycan IGAGI, a fluid-phase marker of Golgi-derived vesicles) was used to measure exocytosis at both ap and bl membranes. At rest, Isc = 3.6, PK = 0.8 x 10-6, Pc0 = 2.1 x 10-' with SPQ and 2.4 X 10-6 electrically, and there was constitutive GAG secretion (i.e., exocytosis) to both ap and bl sides (bl >2 X ap). Carbachol (C) increased: Isc (A = 18.6), PK (6.5X), Pc, ( 1.8-2.9x ), and exocytosis to both ap (2.2-3.5x) and bl (2.0-3.0x) membranes. Forskolin (F) increased ISC (A = 29), PC0 (5.5-11X) and ap exocytosis (1.5-2X), but had no effect on PK or bl exocytosis. Synergistic effects on ISC occurred when C was added to F-treated cells but not vice versa, even though the characteristic effects of F + C on Pc,, PKI and /or GAG secretion were identical to those exhibited when stimulants were added individually. Cl secretion results from coordinated activation of channels at ap and bl membranes, and exocytosis may play a role in these events. (J. Clin.