Abstract: A device for longitudinally adjusting motor vehicle seats, of the type comprising a pair of fixed guides (10) rigid with the vehicle floor and a pair of mobile guides (12) rigid with the seat and slidable along the fixed guides. Between at least one fixed guide and the respective mobile guide there is interposed a spring (44) rigidly fixed to the mobile guide and configured such that under normal operating conditions it engages in toothing (48) on the fixed guide to lock the mobile guide in position. A manually operated lever (54) is provided which acts on the spring (44) to disengage it from the toothing (48) on the fixed guide and release the mobile guide.