A seat longitudinal adjusting mechanism for a motor vehicle seat with a guide rail unit consisting of two rails enclosing therebetween a hollow space and longitudinally displaceably supported at one another. A toothed rack connected with one rail extends within the hollow space, into which engages a pinion supported in a pinion cassette. The pinion is driven by an adjusting gear. For facilitating the assembly, a preassembly rail extending in the rail longitudinal direction is provided which includes fastening means for fastening the pinion cassette at the preassembly rail, independently of the rails, whereby the length of the preassembly rail is essentially equal to the distance of the pinion cassette from one end of the other rail. The preassembly rail preassembled together with the pinion cassette can then be inserted correspondingly into the other rail.