Some reports on an increase of malignant skin tumors following fluorescent lighting made us investigate the systemic effects after exposure to fluorescent rays. Fluorescent rays induce small wavy fluctuations of the plasma concentration of alpha-MSH within normal ranges. Some test persons revealed a steep increase of cortisol values shortly after beginning of fluorescent lighting, indicating that fluorescent rays may induce endocrinological reactions. The significance of these findings with regard to the development of diseases, however, cannot be assessed by now.