Twenty-two normally menstruating women were studied during one menstrual cycle. Blood was collected on 4 occasions and was analysed for free and total cholesterol and triglycerides and phospholipids in the lipoprotein fractions, very-low-density lipoproteins (VLDL), low-density lipoproteins (LDL) and high-density lipoproteins (HDL). Fluctuations in lipid parameters were correlated to serum levels of estradiol-17 beta, progesterone, androstenedione, testosterone and sex-hormone-binding globulin (SHBG). Elevated concentrations of SHBG and HDL-cholesterol and a suppression of LDL-cholesterol were found during the luteal compared to the follicular phase and these findings were interpreted as an estrogenic influence. Consequently the ratio LDL-cholesterol/HDL-cholesterol was depressed during the luteal phase. Triglycerides in serum and VLDL reached a peak at midcycle. When effects on lipid metabolism induced by endogenous steroids were compared to lipoprotein fluctuations exerted by exogenous hormones a parallelism was found in certain variables. However, obvious discrepancies were also found in effects on lipid metabolism, especially for hormones with androgenic properties. The present data underline the necessity of defining in which menstrual phase blood has been collected when lipid metabolism is studied in women of fertile age. Knowledge about metabolic events induced by exogenous sex steroids does not allow conclusions concerning the effects exerted by corresponding endogenous hormones.