Industrial use of induction heaters generating electromagnetic energy of frequencies below 9 kHz makes an exposure of their operators to magnetic field H of high intensity and to electric field E of relatively low intensity. The operators of such heaters have suffered repeatedly from infections in the upper respiratory tract and in the skin of the forearm. The experimental exposure of rats to E or H fields of frequencies of 10 Hz, 50 Hz or 1-45 kHz induced changes in growth rate of exposed animals and weight changes in the pituitary-adrenal glands. These results support the suggestion about the involvement of the endocrine system in the biological reaction to incident electromagnetic fields of VLF and ELF.