Abstract: Apparatus for the dynamic testing of a pair of meshing gear wheels by driving one and restraining the rotation of the other, in which the two gear wheels under test are held in mesh with one another and carried by separate rotary supports comprising pairs of aligned rotary spindles, having means for clamping a wheel under test between facing ends thereof, one spindle of each pair being axially displaceable to effect such clamping and the other spindle of each pair being connected to a rotary electrical machine which can function both as a motor and a brake; accelerometer transducers are mounted on the said one spindle of each pair to rotate therewith and thus with the wheels under test, the output signals from the transducers are fed via slip rings to processing circuits including means for summing and squaring them before passing the processed signals for display.