1. Male C57B1/6J mice bearing Leydig-cell tumours known to synthesize steroids in response to luteinizing hormone (LH) were given intravenous injections of [1,2-³H]cholesterol (50–100μCi per animal). Single-cell suspensions were prepared from the tumours 5–9 days after the injection of [³H]cholesterol and were incubated at 37°C in foetal calf serum supplemented with 50mm-Tris–HCl, pH7.4. At various times after the start of incubation cells were collected by filtration of portions of the suspension and their sterols analysed. Within 10min after LH (5μg/ml) or 3′:5′-cyclic AMP (20mm) was added to the cell suspensions an increased conversion of ester cholesterol into free cholesterol could be demonstrated. 2. To observe this rapid effect of LH it was necessary to incubate the cells for 60min before addition of hormone. 3. The specific radioactivity of testosterone produced was approximately equal to that of the intracellular cholesterol regardless of the presence or absence of LH. 4. The amount of free cholesterol produced in response to LH was far greater than that needed for steroid synthesis. 5. Free cholesterol, but not esterified cholesterol, was released into the incubation medium linearly with time and this release was unaffected by LH. LH may stimulate steroidogenesis in part by increasing the concentration of free cholesterol within the cell.

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