Studies on the growth sequence of follicles in mature mice four to six months old were conducted by giving the three injections of 16 muCi tritiated thymidine (3H-T)/gm body weight at four hour intervals over a period of eight hours. Subsequent radioautographic analysis on ovaries obtaine d 1,3,5,9, and 17 days after the last injection showed an overall follicle growth time of 17 to 19 days. The duration of the different follicle stage times were also estimated from radioautographic data. Intense 3H-T incorporation was noted in the developing antra and zona pellucida of follicles of animals sarcrificed on e and three days after treatment suggesting that these areas serve as precursor storage sites during development. Oocytes of follicles in early antrum formation also showed juxtanucleolar 3H-T incorporation concomitantly with rapid and massive oocyte and follicular growth. The findings further indicate that the earliest follicular cells surrounding the oocyte originate at some distance from the developing primordial follicle.