Previously, it has been proposed that progesterone has an inhibitory effect on leptin secretion by white adipocytes, because female rats treated with progesterone show unchanged plasma leptin concentrations despite heavier fat depots. In this study, we show that adipocytes isolated from intact rats release the same amount of leptin either in the presence or the absence of progesterone in the incubation medium. However, when we isolated white adipocytes from progesterone-treated and sham-treated rats and measured their leptin release for 6 hr, we found that adipocytes isolated from rats treated with progesterone for 72 hr showed a lower leptin release than those of sham-treated rats. These results confirm the proposed inhibitory action of progesterone on leptin production.