By the use of sucrose density gradient and protamine sulfate analyses, the cytosol fraction of rat lung has been found to contain binding activities for both androgenic and estrogenic steroids. These activities are present in lungs of rats of both sexes and at all ages examined. Both activities interacted stereospecifically with, and possessed high affinity for, the respective ligand, and sedimented in the 8-9S region of low salt sucrose density gradients. The androgen-binding activity appeared to bind 5alpha-dihydrotestosterone (DHT) and testosterone with similar affinities, but in vitro, whole lung incubation studies demonstrated that DHT is the perdominant androgen found in lung nuclei. In the immature male rat, removal of endogenous androgen by castration-adrenalectomy had no effect on DHT-binding activity in lung cytosol. In the adult, however, this procedure led to a marked cycloheximide-sensitive increase in binding activity. During postnatal development in male rats, relatively constant levels of both androgen- and estrogen-binding activities were found in the lung until the peripubertal period, from which time a striking increase in DHT binding and a decline in that of estradiol were observed. In female rats, cyclic changes in the level of both binding activities were evident during the estrous cycle. When considered in the light of evidence that sex steroids influence certain aspects of lung biochemistry, these data suggest that the demonstrated binding activities represent hormone receptor activities which may mediate a variety of biological events in the lung.