Radioimmunoassay of FSH and LH serum levels has been carried out on 15 Nigerian women, 6 of them mothers of singletons and 9 mothers of twins (2 of two sets). Higher FSH levels have been found in mothers of twins, which supports the view that a higher production of pituitary gonadotropins, resulting in a higher incidence of multiple ovulation, may be responsible for the higher incidence of twinning in Negro populations.