Esterified and non-esterified (free) cholesterol concentrations were determined in the hamster ovary and isolated corpora lutea during the oestrous cycle, pregnancy and lactation. A significant decrease was observed in the esterified fraction between pro-oestrus and oestrus. The administration of phenobarbital at pro-oestrus prevented ovulation and the fall in the concentration of cholesterol esters. Luteal sterols, in cycling animals, expressed as mg/100 mg ovary were 16·3 to 19·0% and 4·3 to 7·5% of the total ovarian free and esterified cholesterol, respectively.
During pregnancy, concentrations of esterified cholesterol were lower than those found during the cycle. Of the total ovarian cholesterol, 38·7% of the free and 10·3% of the esterified fractions were found in the corpora lutea.
During both the oestrous cycle and pregnancy, a larger proportion of cholesterol was found in the interstitial cells and follicular elements rather than in the corpora lutea. This finding is supported by the high concentrations observed during lactation, when no corpora lutea are formed.