Hamilton (1941:4) found for _Microtus pennsylvanicus_ that macroscopic tubules of the cauda epididymis were an indication of fecundity. By noting the size of the tubules (whether macroscopic or not) and by making smears from them in approximately every 25th male caught, I found that the presence of sperm was positively correlated with large-sized tubules of the cauda epididymis in _Microtus ochrogaster_. Inferentially, males with sperm were fecund. There is a relationship almost positive between the size of the tubules of the cauda epididymis and the length of the testes. Testes longer than 7 mm. have macroscopic tubules in the cauda, and in testes shorter than 7 mm. these tubules cannot be seen with the naked eye, Hamilton (1937b) found that in _M. pennsylvanicus_ testes smaller than 8 × 4 mm. did not contain sperm. The testes of the prairie vole descend into the scrotum in the breeding season. In the two winter months, when the voles did not bring forth young, the testes decreased in size (see figure 3) and were withdrawn into the body cavity. The presence of the testes in the body cavity does not mean that a vole is not in breeding condition, for many specimens with abdominal testes were fecund. The females were considered to be fecund if they were gravid, or if there were placental scars in the horns of the uteri. [Illustration: FIGURE 2. Fecundity of Prairie Voles by Months. Adults and Subadults are Considered Together.] [Illustration: FIGURE 3. Seasonal Changes in the Length of Testes.] SIZE OF LITTERS The number of mammae characteristic of a species of vole may be a rough guide to the average size of a litter for that species. The prairie vole has fewer mammae (three pairs) than some other voles in North America, and might, therefore, be expected to have smaller litters. Fifty-eight gravid females of _Microtus ochrogaster_ examined by me had an average of 3.4 embryos each; the number of embryos ranged from one to seven. Hamilton (1936a) gave 5.07 as the average number of young per litter in _M. pennsylvanicus_. Hatfield (1935) stated that _M. californicus_ has an average of 5.7 young in a litter. Both _pennsylvanicus_ and _californicus_ normally have four pairs of mammae. The expectation as to the size of the litter seems to be realized. In the prairie vole one pair of mammae is pectoral and two pairs are abdominal. Usually a lactating vole showed evidence of only the abdominal mammae having been in use. The size of litters was found to vary with the season of the year (see table 9). Gravid females were collected in every breeding month except September. TABLE 9. _Average Size of Litters of Microtus ochrogaster by Months_[G] Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. 0 2.8 3.9 3.2 3.4 3.1 2.8 3.0 .. 3.2 2.6 0 .. (4) (10) (6) (8) (9) (5) (2) .. (5) (5) ..