The analgesic effect of ceruletide in biliary and renal colic was evaluated by a randomized, double-blind study in 82 patients. Ceruletide was compared with pentazocine, a well-established analgetic agent. Rapid and effective analgesia was obtained by intramuscular injection of ceruletide 0.5 μg/kg in 56 patients with biliary colic. The analgesic effect of ceruletide compared well with pentazocine 0.5 mg/kg im, and was associated with remarkably fewer side effects. In 26 patients with renal colic, ceruletide was significantly inferior to pentazocine. These data support the recommendation of ceruletide as a first-choice analgetic agent for biliary colic.