Follicular fluid (FF) lipoprotein content was evaluated in an in vitro fertilization/embryo transfer and gamete--intrafallopian--transfer program and correlated to follicular and oocyte maturation. Moreover, the in vitro progesterone response of granulosa-luteal cells from 10 patients to high-density lipoprotein (HDL) and to low-density lipoprotein (LDL) was assessed. Most FFs contained only HDL. Sixteen out of 97 FFs contained also very low levels of LDL and very-low-density lipoprotein (VLDL). The presence of LDL was associated to features of follicle and oocyte hypermaturity. LDL alone induced a much more potent increase of progesterone (P) release by granulosa-luteal cells than HDL alone, and the HDL partially reversed the potent effect of LDL. It is concluded that in late follicular phase HDL maintains P release by granulosa cells at a low rate and prevents a potent stimulation of P production by LDL which might cross the maturating blood-follicle barrier, until increasing passage of LDL in FF decreases the HDL:LDL ratio and the action of LDL becomes prominent.