To investigate the effects of hydrocortisone on new-born rat cardiac endothelial cells in culture, the endothelial cells were isolated by means of enzyme-cocktail method. The cells were cultivated in Lees modified Dulbecoulcorner medium and 10[M or 10[M of hydrocortisone was added to the medium. The cells were harvested or coverglass and processed for thiamin pyrophosphatase reaction and Feulgen reaction. The enzymatic activities of Golgi complex, number of cells and number of large nucleated[more than tetraploid] cells were counted and discussed for their significance. The results were summarized as follows; 1. Hydrocortisone seemed to accelerate the rate of recovery of cardiac endothelial cells from isolation damage. 2. Endothelial cells treated with hydrocortisone revealed strong positive reaction to thiamine pyrophosphatase in early culture and 10 M group had stronger reaction than that of 10 AM group 3. Hydrocortisone had inhibiting effects on endothelial proliferation and the higher the concentration of the reagent was the stronger effects. 4. Hydrocortisone inhibited the appearance of large nucleate cells in endothelial cell population. 5. Hydrocortisone seemed to suppress the nuclear DNA synthesis.