Chronic experiment on male rats deals with 3 regimes of the impact of electromagnetic radiation with the frequency of 24 MHz and intensity of 500 V/m. The regimes differ by the duration of radiation exposure. It is established that distinct variations in the analyzed indicators and also their stability significantly depend on the lifetime of daily exposure and grow along with its elevation. The obtained relation of electromagnetic radiation bioeffects with the exposure duration indicates the necessity of taking account of the time variable while setting up hygienic norms for electromagnetic short-wave radiation.