In mammals two meiosis activating sterols (MAS) have been found to activate meiotic resumption in mouse oocytes, in vitro. FF‐MAS (4,4‐dimethyl‐5α‐cholesta‐8,14,24‐triene‐3β‐ol) was extracted from human preovulatory follicular fluid and T‐MAS (4,4‐dimethyl‐5α‐cholest‐8,24‐diene‐3β‐ol) from bull testicular tissue. Quite unexpected, these two sterols, which introduce the cholesterol biosynthetic pathway from lanosterol, may be locally acting substances with important physiological function for reproduction. FF‐MAS and T‐MAS are present in the preovulatory follicular fluid of different mammalian species and have the capacity to initiate resumption of meiosis in mouse oocyte cultured in the presence of hypoxanthine, a natural meiosis maturation inhibitor. FF‐MAS is produced by the cumulus cells of intact oocyte‐cumulus complexes upon FSH‐stimulation and provides the oocyte with a go‐signal for the resumption of meiosis. T‐MAS constitutes the vast majority of MAS found in the mammalian testis and in the human ejaculate; in particular a high concentration is found in the spermatozoa. T‐MAS may be produced by the spermatids and the presence of T‐MAS in spermatozoa may suggest that T‐MAS plays a role in fertilization by affecting the second meiotic division.J. Exp. Zool. (Mol. Dev. Evol.) 285:237–242, 1999. © 1999 Wiley‐Liss, Inc.