I had originally assumed that all adult males would be fertile throughout the breeding season, and that any males with abdominal testes would be subadults or young of the year. This assumption was an oversimplification; all adult males did not reach breeding condition at the same time of year. My data do not support a firm conclusion, for it is difficult to follow non-captive individuals throughout a breeding season, owing to sporadic appearance of animals in traps. Nevertheless, observations of mice that were trapped a number of times indicated the following: 1) Some adult males that had abdominal testes in the middle of July reached breeding condition as late as late August and even late September. 2) Some juvenal males had scrotal testes at the time their postjuvenal molt was just beginning to be apparent on their sides. Most juvenal males did not have scrotal testes, and many juveniles that appeared repeatedly in traps from mid-July through late October did not attain breeding condition. A mouse that was a juvenile in mid-July must have been born in mid-June. 3) Apparently animals born early in the breeding season may reproduce later in that season, whereas those born later in the breeding season tend not to breed until the following year. Possibly cooler evening temperatures in July and August, due to the relatively larger amounts of precipitation in those months, inhibit reproductive development of late-born young. Most plants have ceased vegetative growth and have produced seeds by this time; but the interrelationships between growing seasons, climatic conditions, and reproductive physiology are unknown. Only one adult of each species had scrotal testes after late September; the _P. truei_ had scrotal testes on October 24, 1963, and the _P. maniculatus_ had scrotal testes on October 15 of that year. GROWTH Growth of captive _P. maniculatus_ and _P. truei_ is discussed in several reports. One of the most complete is that of McCabe and Blanchard (1950) on _P. m. gambelii_ and _P. t. gilberti_ in California. A detailed discussion of the dentition in _P. truei_ and wear of the teeth in different age groups is given by Hoffmeister (1951). Molt in these species has been considered by a number of authors (Collins, 1918; McCabe and Blanchard, 1950; Hoffmeister, 1951; Anderson, 1961). The report by McCabe and Blanchard is valuable because molt is compared between the two species from the first to the twenty-first week of postnatal development. [Illustration: FIG. 18: Scatter diagram of postnatal growth of captive mice, showing increase in length of bodies from birth to 70 days of age. The records for _P. truei_ represent 11 individuals of five litters; those for _P. maniculatus_ represent 17 individuals of four litters.]