The reliability of anaerobic threshold and the validity of its use as an index of circulorespiratory endurance and performance capacity were investigated using coefficients of correlation and mean scores. Reliability was determined for anaerobic threshold prediction using nonlinear changes in the V̇e exercise intensity curve and for indices associated with anaerobic threshold. The validity of anaerobic threshold was established through comparison of indices related to recognized trials of functional capacity and endurance performance. Significant coefficients of reliability were produced for predicting AT from nonlinear V̇e responses. In addition, respiratory indices associated with anaerobic threshold were also found to be reliable measures of function. The validity of anaerobic threshold indices was specifically related to functional measures described by performance capacity measurements. There was no evidence to suggest that anaerobic threshold represented measures of functional capacity. It was concluded that an aerobic threshold represented a reliable measure of optimum aerobic utilization which closely followed functional measures associated with performance capacity. From results, several implications were posed for the use of anaerobic threshold. First, anaerobic threshold appeared to identify the metabolic components of exercise. Second, its use reflected a functional measure of optimum aerobic utilization. Third, it reflected exercise demand that was less strenuous resulting in fewer physiological and psychological limitations. Fourth, it implied that performance could be evaluated with respect to aerobic/anaerobic functions. A description of the investigation and the comparisons used for the determination of anaerobic threshold reliability and validity are included. / Master of Science