The effect of STa enterotoxin on intracellular calcium in primary cultures of rat enterocytes was investigated. The enterocytes were loaded with a fluorescent calcium indicator, indo-1, and changes of intracellular calcium determined by flow cytometry. When the rat enterocytes were treated with EGTA, a calcium chelator, or ionomycin, a calcium ionophore, a decrease or increase, respectively, in free intracellular calcium was demonstrated. The addition of STa enterotoxin to rat enterocytes loaded with indo-1 caused an initial decrease, followed by a rapid increase, in free intracellular calcium. These effects on intracellular calcium occurred prior to or during an increase in the intracellular concentration of cyclic 3',5'-guanosine monophosphate. The current results demonstrate that the activation of rat enterocytes by STa enterotoxin has an effect on transmembrane regulation of the receptor-dependent calcium signal and/or the release of calcium from intracellular storage sites.