Pulmonary surfactant, consisting predominantly of phosphatidylcholine (PC), is secreted from Type II cells into the lungs of all air-breathing vertebrates, where it functions to reduce surface tension. In mammals, glucocorticoids and thyroid hormones contribute to the maturation of the surfactant system. It is possible that phylogeny, lung structure, and the environment may influence the development of the surfactant system. Here, we investigate the ontogeny of PC secretion from cocultured Type II cells and fibroblasts in the sea turtle, Chelonia mydas, following 58, 62, and 73 d of incubation and after hatching. The influence of glucocorticoids and thyroid hormones on PC secretion was also examined. Basal PC secretion was lowest at day 58 (3%) and reached a maximal secretion rate of 10% posthatch. Dexamethasone (Dex) alone stimulated PC secretion only at day 58. Triiodothyronine (T(3)) stimulated PC secretion in cells isolated from days 58 and 73 embryos and from hatchling turtles. A combination of Dex and T(3) stimulated PC secretion at all time points.