The effect of various dosages of caffeine upon maximal endurance capacity in females was studied. The effect of caffeine upon resting heart rate (RHR), submaximal heart rate (SHR), maximal heart rate (MHR), and ratings of perceived exertion (RPE) was also studied. Prior to undertaking a standarized progressive workload to exhaustion on an electric bicycle ergometer, each subject consumed either a placebo, small (4 mg/kg), medium (7 mg/kg) or large (10 mg/kg) dose of caffeine. A double-blind procedure was followed. Mean times to exhaustion for the placebo, small, medium and large dosages of caffeine were, respectively, 299.5, 312.1, 299.8, and 303.2 seconds. MHR were respectively, 183.4, 185.0, 185.4, and 184.4. Maximal RPE were 16.6, 17.0, 16.3, and 17.1. The analysis of variance revealed no significant differences between the drug trials for the above three variables; in addition, no significant differences were noted for the RHR, SHR, and submaximal RPE. Within the limitations of this study, the following conclusion appears warranted: A small, moderate, or large dose of caffeine exerted no significant effect upon maximal endurance time, RHR, SHR, MHR, or RPE at either submaximal or maximal workloads.