The smallest female having a vaginal orifice weighed 160 grams, but in most instances somewhat larger size is attained before the onset of oestrus. Judging from the average growth rate of immature females (Fig. 3), most probably attain sexual maturity at an age of five to six months unless this age is reached in the winter period of sexual quiescence. Rainey (_op. cit._) found no clear cut instances of young maturing in time to breed before their first winter. He concluded, tentatively, that in most instances sexual maturity is not attained until the spring of the year following that in which the rat is born. However, the evidence was inconclusive because few of the young marked survived to maturity. In late summer and early autumn, the latter third of the breeding season, newly matured young of the year, born in early spring, may be the most productive group. Young conceived at the beginning of the breeding season, and born in early March, would normally reach adult size and breeding maturity in August. For example, a young female first caught on June 15, 1951, weighed only 150 grams, but by August 10 she had gained to 220 grams (probably in pregnancy) and had a vaginal orifice. Of 35 adult and subadult females examined by Fitch in October, eleven had a vaginal orifice, the latest on October 18. Of these eleven showing signs of breeding, four at least had not yet produced litters, judging from the undeveloped condition of their mammae, and others that showed evidence of recent lactation probably included young of the year that had bred in August or September. One female gave birth to a litter in a trap on the night of October 6, 1950. Of 32 adult and subadult females recorded by Fitch in November, all were sexually quiescent, with the possible exception of one having a partially open vagina on November 10. All females taken in December, and most of those taken in January, also were sexually quiescent. January 20 was the earliest recorded date for a female with a vaginal orifice. Females examined in February mostly were perforate and many of them appeared to be in oestrus. One female trapped on February 19, 1950, weighed only 140 grams and was still imperforate. Another, weighing 200 grams on February 3, 1952, still was imperforate, but by February 27 she was perforate and appeared to be in oestrus. An adult female that appeared to be in oestrus on February 3, 1952, was imperforate on February 10. _Growth_