The uptake of progesterone in the intact female rat brain was studied in 25, immature, female rats which were given an injection of 4 IU PMSG on the 25th day of life. The rats were killed 53-55 h after their first ovulation. Amounts of progesterone were measured in seven brain areas, peripheral fat and muscle tissue and blood plasma. The analyses were done by radioimmunoassay after extraction with ethanol or diethylether. The highest mean concentration was found in the cerebral cortex, followed by the hypothalamus, the hippocampus, the striatum, the midbrain, the cerebellum and the medulla oblongata (mean +/- SE pg/mg 29 +/- 2.0, 27 +/- 5.2, 19 +/- 2.4, 15 +/- 1.8, 14 +/- 2.0, 12 +/- 1.7 and 11 +/- 1.9). The concentration of progesterone in the cerebral cortex was significantly higher than in all other areas except the hypothalamus (p less than 0.001). The cerebral cortex, the hypothalamus and the hippocampus had progesterone levels that were significantly higher than in peripheral fat tissue (p less than 0.01). The plasma progesterone level correlated positively with that in the cerebral cortex (r = 0.62, p less than 0.01). Some earlier studies have shown high accumulation in the hypothalamus, but low in the cerebral cortex. One reason for the different results might be the nonphysiological models used in the earlier studies.