Testes diameter of 110 Merino ram lambs was measured at 100, 260 and 550 days of age. At 80 days of age the LH response to exogenous GnRH was determined in these rams. During the breeding season at 18 months of age the rams were subjected to libido tests. The repeatability of the libido tests was high (0.74 Â± 0.030) and it was evident that rams could be divided into a group displaying a high and a group displaying a low libido. Basal plasma LH concentration as well as plasma LH concentration 30 and 50 min following GnRH administration did not differ between the high and low libido groups. Although the LH response to GnRH was higher (p < 0.05) in the high libido rams (8.33 Â± 0.48 ng /ml) compared to the low libido rams (6.32 Â± 0.76 ng /ml), libido as determined in this study could not be accurately predicted from LH response. Contrary to expectation the testes size at 100 and 550 days of age did not differ between the high and low libido groups.