Transcellular calcium transport by the internally perfused Calliphora midgut has been measured by simultaneously monitoring 45Ca removal from the perfusing saline (entry to the cells) and its appearance in the bathing saline (exit from the cells). Reduction of the Na+ gradient across the basolateral membranes of midgut epithelial cells by removal of bathing Na+ or by addition of monensin or ouabain inhibits calcium transport across the basolateral membranes. Calcium entry at the apical membranes is inhibited in parallel. The calmodulin inhibitors, trifluoperazine or calmidazolium, do not directly affect calcium transport nor do they dissociate the parallel changes in calcium entry and exit when calcium exit is inhibited. Experiments with A23187 are consistent with a role for intracellular calcium in regulating calcium entry at the apical membranes. It is suggested that calcium transport out of midgut epithelial cells is largely by Na+-Ca2+ countertransport, and that entry may be regulated by cytoplasmic calcium so that the calcium influx never exceeds the capacity of the transport mechanisms to pump it out of the cells.