PURPOSE: To completely balance the various forces acting on a modulator mechanism for actuating rotary dobbies for looms.

CONSTITUTION: The modulator mechanism is a modulator mechanism for actuating the rotary dobbies for the looms, which consists of a casing 31 having at least two oscillating supports 37 turned by a shaft 34 of the loom and in which each of the supports has two rollers 36 rolling in contact with a stationary complementing cam 35, and these rollers 36 are connected to a main shaft of the dobby by connecting rods 38 which are connected, on one hand, to the supports and are connected, on the other hand, to cross members cooperating with the main shaft. The connecting points of the connecting rods 38 on the corresponding supports 37 align to a shaft 37a mounting to one of the two rollers 36 of the supports.