An automotive seat adjustment and locking mechanism provides a rack (217) and pinion (229) seat track assembly (100) in which stepless position-adjustment is provided by rotary drive to the pinions either manually or by means of drive. The manual drive incorporates a stepless mechanical lock (210) for defining a seat-adjusted position. The pinion (220) of the seat track assembly is of ring format and has a roller-type locking mechanism (224) at its inner periphery (222) which is releasable by a lever mechanism (238, 240). This locking mechanism is located between the side plates (112, 114) of the seat slide as an integral part of the slide whereby strength is greatly increased. Similar locking facilities may be provided for backrest rake and seat height. Embodiments use a spring lock (228, 230) and a roller lock (224, 226). In one embodiment under overload (impact) conditions a shear pin in the drive allows tha rack and pinion ring gear to shift causing pivoted locking arms to move to locking positions thereby taking over the locking function and avoiding the need for massive strength in the drive train.