3. Quantitative studies on the frequency of chromosome doubling in corn seedlings treated at different temperatures for varying periods of time indicate that 20, 40, and 80 minute treatments at 36°, 38°, 40° and 42° C are effective in producing a markedly increased frequency of tetraploid sectors in the root-tips and stem-tips, more mutant sectors being produced in the roots than in the stems of the same treatment. Negative results were obtained from a study of the persistence in the mature plants of tetraploid sectors induced by heat treatment of the germinating seed. Over 300 plants were included in the experiment and no tetraploid ears or ears with tetraploid sectors, as determined by applying pollen from tetraploid plants to the treated plant and noting the set of seed, were obtained.