1. Using dry seeds of a species the radio-sensitivity of mint seeds was compared by measuring the seedling height at 50 days after X-rays or γ-rays irradiation. 2. The plant height produced fromγ-rays seeds showed a small variation around its mean value, while in X-irradiation there was a greater deviation from the mean value on the growth inhibition. 3. The frequency of mutation was determined by inspecting the appearance of chlorophyll deficiency in a sown strain. 4. The induce more mutants by usingγ-rays than X-rays. 5. A remarkable deviation between the frequencies of mutation obtained in the first time and those in the second. 6. As X-rays and γ-rays are used to obtain the frequency of mutation of a certain value. γ-rays induce more sterility than X-rays provided the used dosage of X-rays is relatively low. 7. In this experiment carried out with mint seeds it was also found the elimination processes occurred less effectively in the case of r-rays than X-rays. 8. The frequency of chlorophyll mutation was influenced by the time of sowing of X2 seeds, especially in xantha type.