The ovulation rate of the mouse was reduced by twice daily injections of small doses of pregnant mares' serum gonadotrophin (pmsg). A dose of 0·5 i.u. pmsg per injection had the greatest effect, and reduced the number of eggs shed at oestrus and in response to hcg by 3·5 eggs (26%) and 4·7 eggs (34%), respectively. This effect was greater than that of single doses.
The equilibrium level of pmsg which produced the greatest reduction in the ovulation rate was estimated to be 1·5 i.u.