Air leakage in underground mines is the most common cause of inefficient distribution of ventilating air. Depending upon various factors, more than half of the fresh air entering a mine short-circuits to the return airways before reaching the working faces. Consequently, large leakages create an additional demand for increased intake air, which substantially affects the efficiency of a mine ventilation system. In this study, two aspects of the leakage problem have been addressed. The first involved the difficulty of measuring air leakage. Air leaks are traditionally many in number and not necessarily large in quantity at any one source. Thus, the measurement of low air leakage quantities is difficult. The most sensitive, permissible, air velocity measuring equipment available can not detect the majority of air leakage. To aid in the detection of air leakage, a portable leakage measuring device was developed. A discussion concerning the device's design, practicality, laboratory results, and field testing is presented. The second area of research investigated the effect of leakage on a system's fan power consumption. A mine model was used to simulate different leakage conditions and monitor the corresponding fan power requirements. The results showed a number of very interesting trends. The model also pointed out the different variables that affect the influence of the leakage rate on the power consumption. The discussion of these findings is included. / M.S.