The frequencies of sex-linked recessive lethal mutations in F 1 males after feeding adult male Drosophila melanogaster with 0.25 and 0.5 mM methyl methanesulfonate (MMS) orally for 24 h increased approximately linearly with storage of the treated spermatozoa in females, whereas the number of hits of dominant lethals in the sperm after feeding 0.3 and 0.5 mM MMS increased approximately with the square of the storage time. Chromosome losses and mosaics in F 1 males also increased with the dose of MMS to males, but their yields were too low to be analyzed quantitatively, only indicating a slight increase of chromosome losses and a slight decrease of mosaics with the time of storage of sperm. Maternal non-disjunctions (or chromosome losses), detected in F 1 males, decreased with the dose of MMS to spermatozoa and their yield decreased with the time of storage of sperm of both MMS-treated and the control groups. A unitary model is proposed to explain the effect of storage on the dominant lethals and recessive lethal mutations.