OBJECTIVES: To report the physiological responses of indoor rock climbing. METHODS: Fourteen experienced climbers (nine men, five women) performed three climbing trials on an indoor climbing wall. Subjects performed three trials of increasing difficulty: (a) an easy 90 degrees vertical wall, (b) a moderately difficult negatively angled wall (106 degrees), and (c) a difficult horizontal overhang (151 degrees). At least 15 minutes separated each trial. Expired air was collected in a Douglas bag after four minutes of climbing and heart rate (HR) was recorded continuously using a telemetry unit. Arterialised blood samples were obtained from a hyperaemised ear lobe at rest and one or two minutes after each trial for measurement of blood lactate. RESULTS: Significant differences were found between trials for HR, lactate, oxygen consumption (VO2), and energy expenditure, but not for respiratory exchange ratio. Analysis of the HR and VO2 responses indicated that rock climbing does not elicit the traditional linear HR-VO2 relationship characteristic of treadmill and cycle ergometry exercise. During the three trials, HR increased to 74-85% of predicted maximal values and energy expenditure was similar to that reported for running at a moderate pace (8-11 minutes per mile). CONCLUSIONS: These data indicate that indoor rock climbing is a good activity to increase cardiorespiratory fitness and muscular endurance. In addition, the traditional HR-VO2 relationship should not be used in the analysis of this sport, or for prescribing exercise intensity for climbing.