The concentrations of total cholesterol, phospholipids, triacylglycerols and lipoproteins were measured in 87 follicular fluids obtained from 35 women undergoing in vitro fertilization and embryo transfer. The results were correlated with the levels of progesterone in follicular fluid. Two different types of ovarian stimulation were used. High density lipoproteins were the dominant lipoproteins found in the preovulatory follicular fluid. Low density lipoproteins were absent or appeared in trace amounts. Significantly higher triacylglycerol and high density lipoprotein levels were found when stimulation with human menopausal gonadotropins and chorionic gonadotropin was applied, as compared to the clomiphene citrate-menopausal gonadotropin-chorionic gonadotropin menstrual cycle. In both groups, extracorporal fertilization resulted in cleavage of oocytes and embryo transfer. No significant correlation between any follicular fluid lipid and progesterone concentration was found. The lipids estimated in the follicular fluid appeared to have no influence on the oocyte fertilizability. The presence of triacylglycerols and high density lipoproteins in the follicular fluid may indicate follicular wall permeability under the treatment with menopausal gonadotropins.