Gonadotrophin Releasing Hormone (GnRH) is secreted in a pulsatile manner by the hypothalamus. GnRH is the major hormone controlling the pituitary-testicular axis and therefore influences aggressive and sexual behaviour in bulls. In 6 to 10 week old bull calves an increase in GnRH pulse frequency is responsible for a short-lived rise in circulating levels of LH. It has been shown that bulls with a higher rise in LH attain puberty at younger ages and have comparatively enhanced semen quality once they mature (Evans et al., 1995). Furthermore testicular growth has been enhanced in calves with a premature increase in LH brought about by GnRH treatment (Chandolia et al., 1997). This experiment tests the hypothesis that bull calves with increased GnRH pulsatility engage in more male-male mounting behaviour during this early period while the pattern of hormone secretion is becoming established. Studying this behaviour may give an indication of reproductive potential from as early as 8 weeks of age.