In 4 separate experiments involving 133 ewes the timing of the onset of the LH discharge relative to the onset of oestrus was compared in prolific Booroola Merinos and in other Merinos of low ('C' and 'O' genotypes) and medium ('T' genotype) prolificacy. Despite different average ovulation rates of 3.05, 1.85, 1.18 and 1.37 for Booroola, 'T', 'O' and 'C' ewes, respectively, there were no consistent differences in the time of the LH discharge which occurred on average 4.5 h after the onset of oestrus. Time of ovulation was studied, in two experiments involving 430 Booroola, 'T', 'O' and 'C' ewes, by laparoscopic ovarian examination of subsamples of ewes at 20, 24, 28 or 32 h after onset of oestrus. Ovulation occurred significantly (P less than 0.001) earlier (7.5 h) in Booroola ewes than in those of the other genotypes.