Pulmonary alveoli, especially in females, are estrogen responsive structures: ovariectomy in wild type (wt) adult mice results in alveolar loss, and estradiol replacement induces alveolar regeneration. Further, estrogen receptor (ER)-alpha and ER-beta are required for the developmental formation of a full complement of alveoli in female mice. We now show ovariectomy resulted in alveolar loss in adult ER-beta-/- mice but not in adult ER-alpha^-/- mice. Estradiol treatment of ovariectomized ER-beta^-/- mice induced alveolar regeneration. In ovariectomized wt mice, estradiol treatment resulted, within one hr, in RNA-level gene expression supportive of processes needed to form an alveolar septum, e.g., cell replication, angiogenesis, extracellular matrix remodeling, and guided cell motion. Among these processes, protein expression supporting angiogenesis and cell replication was elevated one hr and 3 hr, respectively, after estradiol treatment; similar findings were not present in either mutant. We conclude 1) loss of signaling via ER-beta is not required for post-ovariectomy induced alveolar loss or estradiol induced regeneration; this indicates ER-alpha is key for estrogen related alveolar loss and regeneration in adult female mice; 2) taken with prior work showing developmental formation of a full complement of alveoli requires ER-alpha and ER-beta, the present findings indicate the developmental and regenerative formation of alveoli are regulated differently; i.e., signaling for alveolar regeneration is not merely a recapitulation of signaling for developmental alveologenesis; 3) the timing of estradiol-induced gene expression in lung supportive of processes required to form a septum differs between ovariectomized wt and ER-^-/- mice.