Testes of mature male mice were irradiated with a single dose of 1000 rads. Enumeration of the Sertoli cells at different intervals showed an increase in their number per mm3 of tubule, but after the application of correction factors there was an immediate fall in the total Sertoli cells per testis which later stabilized at about half the preirradiation level. There was a reduction in the mean number of chromocentres per Sertoli cell, owing to a relative increase in the number of nuclei with one chromocentre, 14 days after irradiation.
It was further seen that labelled spermatocytes were transformed to restitution cells at the time that unirradiated spermatocytes would have produced spermatids. Seven days later the label appeared in spermatozoa. This implied that the restitution cells were not eliminated but were transformed to spermatozoa and the time-table of events was similar to that in normal spermiogenesis. No label appeared in Sertoli cells.
The number of grains present in spermatocytes and restitution cells and spermatozoa was nearly the same, indicating no intervening nuclear divisions.
Enumeration of the number of surviving labelled cells confirmed preleptotene as the most sensitive stage of spermatocytes.