Although females of _M. ochrogaster_ are much less precocious in their manifestations of puberty, they may become perforate well before impregnation can occur, and seem to pass through sterile cycles before becoming pregnant. The 18 smallest females recognized as being pregnant were of the following over-all lengths, in mm.: 149, 149, 149, 148, 148, 148, 147, 146, 145, 145, 144, 144, 143, 143, 143, 142, 135, and 134. As pregnancy is ordinarily recognized only in the last four days the females must have been impregnated from 20 to 17 days earlier--when they were in most instances 7 to 11 weeks old and 135 to 145 mm. in length. The two smallest individuals, recorded as pregnant at 135 and 134 mm., must, if they were of typical size for their age, have become pregnant at an age of approximately one month, when they were only 119 and 122 mm. in length. The smallest lactating females (some of them pregnant also) were recorded at lengths of 149, 148, 148, 147, 147, 146, 144, 144, 143, 143, and 142 mm. Occasionally females of less than 120 mm. were found to be perforate, and seemingly had begun oestral cycles. Records of a female of definitely known age, typical of many of the same size in her development, are cited below: March 19, 1956 Born in captivity. April 7, 1956 (19 days old) Released on study area at site of mother's capture; length 102 mm., weight 11.1 gms. April 15, 1956 (27 days old) Recaptured; perforate with a copulatory plug; length 113 mm., weight 13.4 gms. April 27, 1956 (39 days old) Recaptured; imperforate; length 131 mm., weight 24.3 gms. May 12, 1956 (54 days old) Recaptured; perforate and in late pregnancy; length 146 mm. May 25, 1956 (67 days old) Recaptured; imperforate, in an advanced state of lactation; length 150 mm., weight 33 gms. [Illustration: FIG. 7. Growth curve in the prairie vole; dots are based on means of series of definitely known age (born in captivity); circles are based on mean lengths of recaptured marked young whose ages were not precisely known.] [Illustration: FIG. 8. Over-all length in young prairie voles of definitely known ages, up to 40 days. All were born in captivity. Some were released with the female and developed under natural conditions, but their growth rate did not differ discernibly from that of those kept in the laboratory. Dots indicate individual records; circles are means for ages at which four or more records were obtained.] When captured on May 12, at an age of 54 days, this female appeared to be within two or three days of parturition, and hence must have become pregnant at an age of approximately 35 or 36 days. Pregnancy in the more precocious females probably occurs at a length of approximately 130 mm. and an age of a little less than 40 days. Such females are still growing so rapidly that by the time their litters are born, they have grown to more than 140 mm. GROWTH IN SUBADULTS AND ADULTS