The effect on mammary explants of the addition of insulin and cortisol, alone and in combination, to the synthetic culture medium "199" was studied. Concentration of insulin was about 140 [mu]g/ml and cortisol 8 [mu]g/ml. Normal prelactating explants were derived from C3H mice during the 14th-17th day of pregnancy. Hyperplastic alveolar mammary nodules were also used. The alveoli of these tissues normally show histologic evidence of secretory activity. In "199" alone, or in "199" containing added cortisol, explants showed extensive alveolar degeneration. In media containing cortisol + insulin, the alveoli were maintained, but showed no intracellular vacuoles or stainable secretion in the lumina. Insulin alone also improved the survival of the explants, but not so well as those cultured with both cortisol and insulin. In contrast to the results with cortisol + insulin, cortisol + mammotropin[long dash]treated alveoli contained many intracellular vacuoles and a dense, darkly-stained secretion in alveolar lumina.