Variability in superovulatory responses remains one of the major factors preventing the more widespread utilisation of embryo transfer in sheep breed improvement. Previous work in our group has suggested that in sheep receiving a standard superovulatory treatment regime using oFSH (Ovagen, Immuno-chemical Products Ltd., Auckland, NZ) there is an identifiable pool of follicles at the start of oFSH treatment that eventually ovulate following treatment. If this is the case then research efforts to increase the consistency of superovulatory responses should be focused on increasing the supply of follicles in this category. This experiment therefore had two aims, i) to test the theory that variability in superovulatory responses is a function of variability in the number of potentially ovulatory follicles at the time of the first FSH injection, and ii) to identify the size category of follicles at the start of superovulatory treatment that forms this ovulatory pool.