The existing methods of estimating the actual loads on the teeth of gears at speed are by no means satisfactory. Discrepancies between the methods are illustrated by calculations made for typical conditions to be used in the tests. A discussion on the possible factors that may influence the dynamic load and on the practical value of an exact knowledge is included. A power circulating rig that is being built at the College for the measurement of the dynamic loads is described together with the proposed method of instrumentation. The main design features of this rig are its accuracy under loading conditions, its slip free loading coupling and its adjustable centre distance. The proposed research programme includes tests on spur and helical gears both with and without profile modification. Investigations will be carried out on these gears at pitch circle velocities over the range 0- 20,000 ft. /min. and tangential tooth loads of 0 - 3,000 lb. /in. face.