Background: Systemic administration of a cholinergic blocking agent or glucagon is used to reduce spasms, but it is inconvenient and sometimes causes side effects. This study is an evaluation of the intracolonic administration of peppermint oil during colonoscopy for the control of colonic spasm. Methods: Each patient in the treated group (n = 409) was given approximately 200 mL of the solution (a mixture of 8 mL of peppermint oil and 0.2 mL of Tween 80 per 1 L of water with 0.04% indigo carmine) by using a hand pump attached to the accessory channel of the colonoscope. Changes in patient posture were made to distribute the solution. The patients in the control group (n = 36) were given the solution without peppermint oil. Results: A satisfactory spasmolytic effect was seen in 88.5% of the treated patients and in 33.3% of those in the control group (p < 0.0001). No adverse effect was observed. The mean time to onset was 21.6 ± 15.0 seconds, and the effect continued for at least 20 minutes. In patients with irritable bowel syndrome, efficacy was significantly lower (p < 0.0001). Conclusions: The intraluminal administration of peppermint oil by using a hand pump is a simple, safe, and convenient alternative to the systemic injection of a cholinergic blocking agent or glucagon during colonoscopy. (Gastrointest Endosc 2001;53:172-7.)