1. The effect of cortisol and thyroxine on the formation of phosphatidylcholines and phosphatidylglycerols was studied in adult rat lung type II cells in primary culture. 2. Addition of cortisol enhanced the incorporation of [Me-14C]choline, [1-14C]acetate, [1-14C]palmitate, D-[U-14C]glucose, and [1(3)-3H]glycerol into total and disaturated phosphatidylcholines. 3. Cortisol also stimulated the formation of phosphatidylglycerols from labelled acetate, palmitate, glucose, and glycerol, but did not affect the formation of phosphatidylethanolamines. 4. Thyroxine alone did not significantly affect the formation of total and disaturated phosphatidylcholines nor that of phosphatidylglycerols or phosphatidylethanolamines. 5. Exposure of the cells to a combination of cortisol and thyroxine caused increases in the rates of synthesis of total and disaturated phosphatidylcholines from labelled choline, palmitate, and glycerol and in that of phosphatidyl-glycerols from labelled glycerol. These increases were about the same as those brought about by cortisol alone. In contrast to cortisol alone, the combination of cortisol and thyroxine did not significantly affect the entry of labelled acetate and glucose into phosphatidylcholines and phosphatidylglycerols. 6. The present results suggest that direct effects of glucocorticosteroids on the alveolar type II cell may play a role in the regulation of the synthesis of surfactant lipids in the adult lung.