Endocrine control of cytoplasmic factors modulating adenylate cyclase activity in rat lung membranes was investigated. Hypophysectomy, adrenalectomy and thyroidectomy showed an adverse effect on the body and organ weights. Lung protein, glycogen and DNA contents were decreased in the endocrine ablated animals which were restored to the normal values on hormone treatment. Phosphodiesterase and phosphorylase activities were increased and decreased in adrenalectomized and thyroidectomized animals, respectively. The activities of these enzymes were restored to normal values on hormone treatment. Adrenalectomy and thyroidectomy affected ATPases differently. Basal adenylate cyclase activity in rat lung membranes was not affected by adrenalectomy and hormone treatment. However, the total enzyme activity was increased by both dexamethasone (DEX) and thyroxine (T4) treatments. The activation of the particulate adenylate cyclase by the cytoplasmic factors was markedly decreased in the lung from hypophysectomized, adrenalectomized and thyroidectomized rats. This decrease in the cytoplasmic activation of adenylate cyclase was restored to or above the control values on hormone treatment. Alteration in the activation of enzyme by cytoplasmic factors did not appear to be due to the change in the responsiveness of the enzyme. Glucocorticoids appeared to have a specific effect on the cytoplasmic factors modulating the enzyme.