An obese, hypertensive 39-year-old man without known heart disease died in ventricular fibrillation immediately after completing a roller coaster ride, prompting this investigation of the heart rate response to fright stress. Ambulatory electrocardiogram recording was utilized in a study of 10 apparently healthy volunteers, aged 24 to 43 years, during 34 rides on a 'corkscrew' roller coaster. In order to separate the effects of fright from possible changes in heart rate resulting from changes in posture endured during the ride, an additional subject was monitored on a 12 hour marathon ride, consisting of 351 revolutions; it was felt that the fright element would resolve as familiarity increased. The average peak heart rate recorded in the study group was 138 b.p.m. and the highest heart rate was 170 b.p.m. Four subjects achieved 80 to 94% of their age-adjusted predicted maximal heart rate, and six achieved 63 to 74%. The older subjects tended to reach higher rates. Arrhythmias were uncommon and there were no abnormal S-T segment shifts. The marathon rider had a significant heart rate response to the first three revolutions, then more moderate responses for the remainder of the experience, implicating fright as the causative agent. It is suggested that this type of recreational fright stress can precipitate significant heart rate elevations, and possibly should be avoided by the large group of individuals at high risk of occult coronary artery disease.