The concentrations of the radioactive nuclides 210Pb, 212Pb, and 218Po were measured in the samples sequentially taken in three thunderstorms which occurred at Fayetteville, Arkansas, on April 18, June 9, and July 25, 1974. The concentrations of 212Pb were found to vary from 30¿10 12Ci/1 to 352¿1012 Ci/1 and the 212Pb/218Pb ratio, from about 11 to 50 during the rains sampled. In all three rains sampled, the 212Pb/218Pb ratio increases to a maximum during the intense period of the storm and decreased sharply as the storm dissipated. The observed activity ratios during the rainstorms are compared with those measured in light showers and air filter samples. A discussion is presented which explains the variations in the lead isotope ratio as resulting from turbulent mixing of air masses within thunderstorms.