Cells acquire cholesterol through endogenous synthesis and through receptor-mediated uptake of cholesterol-rich low density lipoprotein (LDL). Esterification of LDL-derived cholesterol is catalyzed by acyl-CoA:cholesterol acyltransferase (ACAT) in the endoplasmic reticulum (ER). Progesterone inhibits esterification, and, although the mechanism of inhibition is not completely understood, this inhibition results from progesterone's ability to inhibit the activity of multiple drug resistance (MDR) P-glycoproteins (P. DeBry and J. E. Metherall, submitted for publication). In the current manuscript, we demonstrate that progesterone inhibits cholesterol biosynthesis resulting in the accumulation of a number of sterol precursors. In Chinese hamster ovary (CHO) cells, high concentrations (100 microM) of progesterone completely blocked cholesterol production, resulting in the accumulation of lanosterol and a lanosterol precursor. Lower concentrations (40 microM) of progesterone cause plasma membrane accumulation of several sterol products. The majority of these sterols are precursors of cholesterol since they were efficiently converted to cholesterol upon removal of progesterone from the culture medium. Although very high concentrations (> 200 microM) of progesterone killed CHO cells, their growth was restored by the addition of cholesterol to the growth medium, indicating that progesterone toxicity resulted from cholesterol auxotrophy. The effect of progesterone was not unique to CHO cells; progesterone also inhibited cholesterol biosynthesis in all human cell lines tested. These observations suggest that a common progesterone-sensitive pathway is involved in both cholesterol biosynthesis and the processing of LDL-derived cholesterol.