The Bureau of Mines conducted an experiment at Exxon MineralCompany's Buffalo shaft operation to determine the time required for radon concentrations to return to a base average after fan shutdowns. Shutdown times ranging from 5 min to 16 hr were evaluated. An equation was derived to calculate a ratio that determines the time required for the radon concentration to return to the average level that existed in the mine prior to shutdown. For fan shutdowns of 15 min or more, underground personnel should be evacuated to areas with natural downcast ventilation. For shutdowns of less than 1 hr, personnel should not return to the working area until the elapsed time is four times the shutdown, or until the working area is resampled and found safe. If the mine is not in production during weekends, the downcast fans can be turned off from midnight Friday until 4 p.M. Sunday, and the radon daughter concentration should be below 1 working level throughout the mine by the start of the shift Monday. This fan shutdown would conserve energy and save the mining company an estimated $21,000 each year.