Thoroughbred horses have been bred exclusively for racing in England since Tudor times and thoroughbred horse racing is now practised in over 40 countries and involves more than half-a-million horses worldwide. The genetic origins of the thoroughbred go back largely to horses imported from the Middle East and North Africa to England in the late seventeenth and early eighteenth centuries. Since the establishment of the Stud Book in 1791, the population has been effectively closed to outside sources, and over 80% of the thoroughbred population's gene pool derives from 31 known ancestors from this early period. Despite intense directional selection, especially on the male side, and the generally high heritabilities of various measures of racing performance, winning times of classic races have not improved in recent decades. One possible explanation for this is that additive genetic variance in performance may have been exhausted in the face of strong selection. To test this, we have estimated the genetic trend in performance over the period 1952-77 using TIMEFORM handicap ratings which are based entirely on the horse's own performance, and express its racing merit as a weight in pounds which the compilers believe the horse should carry in an average free-handicap race. These ratings take into account such factors as the firmness of the ground, the distance and the level of the competition. Our results indicate that the failure of winning times to improve is not due to insufficient genetic variance in the thoroughbred population as a whole.