We used samples of random and selected oat seeds (for heavy weight) from thermal neutron-(TN) and ethyl methanesulfonate-(EMS) derived populations, to initiate a second cycle of mutagenesis with these same mutagens. Oat lines derived directly from the random and selected samples and from populations derived from new mutagen treatments were grown in a replicated field experiment and measured for 100-seed weight. The populations derived from the selected oat seeds had a mean genotypic variance component 41 per cent greater than that in the population derived from the random oat seeds. With a second mutagen treatment, the genotypic variance components were increased by 53 and 64 per cent in the random and selected materials, respectively. Recurrent treatment of selected and random samples with a mutagen reduced the mean 100-seed weight 0·02 and 0·01 g, respectively, both being nonsignificant values. The variant lines produced by the second mutagen treatment of either random or selected material were distributed to both heavier and lighter seed-weight classes. The expected gains from a second cycle of selection were greater than the actual gains in the first.