This study investigated the effects of supplementation of the nutraceutical product Eurycoma longifolia Jack on recreational athletes` endurance running capacity and physiological responses in the heat. Twelve Malaysian healthy male recreational athletes (Age: 23.3 (3.7) years old; VO2max: 45.1 (8.1) ml.kg-1.min-1) were recruited in this double blind, placebo-controlled, cross-over study. Subjects completed two endurance running trials in the heat (31°C, 70% relative humidity), performed on separate days, after consuming either 2 capsules of Eurycoma longifolia Jack (75 mg per capsule) or a placebo per day for 7 days before and one hour prior to the experimental trial. On trial day, after 5 minutes of warm-up at 50 % VO2max, the subjects were requested to run on the treadmill at 60 % VO2max for 60 minutes. This was immediately followed by a 20-minute time trial for determining endurance running capacity. Blood samples were taken before warm up, after warm-up, and every 20 minutes during the trial. Statistical analysis was performed using one-way ANOVA with repeated measures. Results showed that the endurance running capacity of E. longifolia was not significantly different from that of the placebo trial. Similarly, oxygen uptake, heart rate, skin temperature, tympanic temperature, ratings of perceived exertion, haemoglobin concentration, haematocrit level, plasma glucose concentration, and plasma free fatty acid concentration were not significantly different between the trials. These findings suggested that supplementation of the E. longifolia product at a dosage of 150 mg.day-1 for 7 days has not provided beneficial effects on endurance running capacity and physiological responses of recreational athletes in the heat. Higher dosage and longer duration of supplementation of the product may be warranted to evaluate further its endurance capacity during exercise.