The effects of alterations in extracellular calcium concentration on prostaglandin (PGE) and thromboxane (TXB2) syntheses were studied in isolated epithelial cells from the urinary bladder of the toad, Bufo marinus. In epithelial cells prepared using collagenase, basal iPGE synthesis was greater than iTXB2 synthesis. Increasing extracellular calcium from zero to 1 mM increased iPGE synthesis and decreased iTXB2 synthesis equivalently such that total conversion of endogenous arachidonate to these two metabolites was unaltered. Vasopressin stimulated iPGE and iTXB2 syntheses when the incubation buffer contained 1 mM calcium but had no effect in the presence of 0.4 microM calcium. In contrast, using an EDTA isolation method, basal iPGE and iTXB2 syntheses were equal in the presence of zero calcium. Increasing extracellular calcium concentration to 1 mM caused a greater enhancement in iTXB2 synthesis compared to iPGE. Increasing extracellular calcium to 2 mM was associated with a decline in iPGE and iTXB2 syntheses back to the levels observed with no calcium added to the medium. The effect of increasing the calcium concentration was greater in phosphate than in bicarbonate buffer. In a Tris buffer the effect of altered calcium was almost completely abrogated. These studies demonstrate that the choice of buffer and alterations in extracellular calcium concentration differentially alter basal arachidonic acid metabolism to prostaglandins and thromboxane in isolated toad urinary bladder cells. The results suggest that there may exist several endogenous pools of arachidonic acid which are differentially influenced by calcium. Furthermore, the pool sensitive to vasopressin has an absolute requirement for calcium.