In experiments de-signed to determine the duration of the effects of ir-radiation upon mature sperm and to discover whether there is a differential effect of the same dose upon the various stages of spermatogenesis, both radium and x-rays were used. Muller''s CIB method for detecting lethals was the breeding technique employed. It was found that when irradiated cf1 d" are continuously mated those sperm which are mature at time of treatment function for approximately 2 weeks, as indicated by the constant mutation rate, after which there is a sudden and sharp drop in the rate, presumably due to fertilization now being accomplished by sperm which were immature at time of treatment. This leads to the conclusion that there is a differential effect of the rays upon immature and mature d" germ cells.