SUMMARY1. Inorganic pyrophosphate (PPi) and diphosphonates inhibit glucagon‐stimulated and fluoride‐stimulated adenylate cyclase activity of rat liver.2. Concentrations of diphosphonates required to produce inhibition are lower than those of PPi, and PPi inhibited fluoride‐stimulated activity to a greater extent than glucagon‐stimulated activity.3. Diphosphonates have inhibitory activity in the presence and absence of an ATP regenerating system in the adenylate cyclase assay, and do not substantially affect the efficiency of the ATP regenerating system. No evidence was obtained that reversal of adenylate cyclase was responsible for PPi inhibition.4. Fluoride virtually completely inhibits pyrophosphatase activity in crude membrane preparations from rat liver, whereas glucagon has no detectable