Summary. A single, ovulatory dose of 25 μg of a highly purified preparation of ovine FSH caused ovulation in 89% of hypophysectomized and 91% of intact female mice primed 48 h earlier with PMSG; the number of oocytes recovered (29·4 ± 4·7 and 22 ± 2·/mouse ovulating, respectively) compared favourably with the 20·0 ± 2·9 oocytes per ovulating female recovered from animals that received PMSG + hCG. After oFSH injection, 82% of oocytes released were fertilized and developed to blastocysts. That the trace contamination (<0·2%) of the oFSH with oLH was not responsible for the ovulation was shown by the markedly reduced number of oocytes collected from ovulating females that were injected with equivalent low levels of hCG (0·001 μg) or oLH (1 μg) (9·0 ± 3·3 and 8·0 ± 3·1, respectively). These results demonstrate that oFSH is as effective as LH in inducing ovulation of competent oocytes in the mouse.