AbstractCimetidine has recently proved useful in the prophylaxis of certain allergic and other hlstamlne‐medlated reactions [1–6, 25]. We have encountered a patient who had a serious cutaneous and respiratory reaction to Renografln‐76 during cardiac catheterizatlon. For the four days preceding his second catheterizatlon, the patient was treated with prednlsone, 30 mg daily, dlphenhydramlne, 25 mg orally, three times a day, intravenous dlphenhydramlne, 25 mg, three times a day, and Solu‐Cortef, 500 mg, intravenously, every six hours. Despite this preparation, he developed an intense reaction to Renografln‐76, which did not respond to further doses of Solu‐Cortef and dlphenhydramlne in the catheterlzation laboratory. Cimetidine, 300 mg, in 100 ml of D5W over 15 minutes, rapidly reversed the entire cutaneous and respiratory phenomenon.In the future, strong consideration should be given to using Cimetidine, in its intravenous form, as a routine medication in angiographlc laboratories where radio‐graphic contrast materials (ROM) are used. It may be helpful as a pretreatment for recognized RCM reactive patients when catheterlzation