A seat reclining mechanism comprises first and second hinge plates for mounting on a seat and a back-rest respectively, and an internally toothed ring gear on each of the hinge plates. One ring gear has more teeth than the other ring gear, and a drive gear extends within the two ring gears. Further provided are means for engaging the drive gear and the ring gears, which includes at least one planetary gear in mesh with both ring gears and the drive gear, and means for rotating the drive gear to effect relative angular adjustment of the two hinge plates. In another embodiment of the present invention a drive gear is within the space within the two ring gears, and a pair of carrier members sandwich the drive gear therebetween. The carrier members position and locate the one or more planetary gears, and further provide a floating nature to the operation of the mechanism which compensates for wear of the gears.