1. The goal of this study was to determine if an increase in cytoplasmic calcium concentration ([Ca2+]i), in the absence of additional second messengers derived from membrane phospholipid turnover, is a sufficient signal to induce chloride secretion across monolayers of the human colonic epithelial line, T84. 2. Thapsigargin was used to increase [Ca2+]i by inhibiting the endomembrane Ca(2+)-ATPase. [Ca2+]i was monitored in monolayers by fura-2 fluorescence spectroscopy, chloride secretion by measuring changes in short circuit current (Isc) in modified Ussing chambers, and inositol phosphates were measured by radio-h.p.l.c. of extracts of cells prelabelled with [3H]-inositol. 3. Thapsigargin increased [Ca2+]i and Isc in parallel, without increasing any inositol phosphates. The effect of thapsigargin on Isc was abolished by the intracellular calcium chelator, bis-(o-aminophenoxy)-ethane-N,N,N',N"-tetraacetic acid (BAPTA). 4. Increasing [Ca2+]i with thapsigargin did not prevent a subsequent calcium response to carbachol or histamine if extracellular calcium was available. In the absence of extracellular calcium, only one such release of calcium to hormonal stimulation occurred when cells were pretreated with thapsigargin, and a second response to either carbachol histamine was essentially abolished. 5. Addition of carbachol or histamine to thapsigargin-treated cells mounted in Ussing chambers caused a transient further increase in Isc followed by termination of the response, even though [Ca2+]i continued to rise. 6. We conclude that an elevation in [Ca2+]i is a sufficient signal to induce chloride secretion in T84 cells. Rather than being required to stimulate secretory responses, additional second messengers induced by hormonal secretagogues (such as inositol phosphates) may in fact serve to limit the secretory response.