Adenylate cyclase activity of bovine lens epithelial cells is activated by guanine nucleotides or fluoride. Additional activation to that of guanylylimidodiphosphate [Gpp(NH)p] was induced by isoproterenol. Insulin was found to inhibit adenylate cyclase activity. Adenosine at low concentrations (5–50 n M) increased adenylate cyclase activity, while at high concentrations it lowered the activity of the enzyme. The occurrence of R and P adenosine sites in lens epithelial cells is suggested. Like other tissues, lens epithelial cells possess a transduction system (adenylate cyclase-G protein complex) for external messages.