Androgens and estrogens are known regulators of fetal and postnatal lung development, but their levels in the developing lung have never been determined. We present here, for the first time, a gas chromatography-mass spectrometry (GC/MS) quantification of dihydrotestosterone, testosterone, androstenedione, and estradiol in canalicular, saccular, and alveolar stage lungs of both sexes. Testosterone, androstenedione, and estradiol were observed in all the analyzed lung samples from gestation day (GD) 16.5 to postnatal day (PN) 30, totalizing 383 individual mice. Levels of these three steroids decreased between birth and PN 5. In contrast, dihydrotestosterone was detected only in male samples on GD 19.5, PN 0, and PN 30. A significant sex difference was observed for testosterone and androstenedione but not for estradiol. Steroid levels were also determined in skinned hind legs for comparison. Three-way analysis of variance revealed that tissue (lung or leg) had a significant effect on testosterone levels for both sexes, but not on androstenedione and estradiol levels. Low but significant testosterone and androstenedione levels were observed in all the females and in prepubertal male samples. These levels must be sufficient to induce androgen receptor activation, as suggested by our recent report showing the presence of androgen receptor in the nucleus of several lung cells in corresponding developmental ages and sexes.