_P. truei_ and _P. maniculatus_ reproduce at about the same time. A female of _P. truei_ prepared as a specimen on May 10, 1964, contained four embryos, each 20 millimeters long, indicating a breeding time in mid-April. Svihla (1932:19) reported the gestation period for non-lactating _P. truei_ to be 25 to 27 days and for lactating individuals, 40 days. Lactation tends to increase the gestation period of other _Peromyscus_ by about five days (Asdell, 1964:266). The gestation period of nine non-lactating females of _P. m. rufinus_ was reported by Svihla to be 23 to 24 days. Lactation increased the length of the period of gestation in this subspecies to between 23 and 32 days (mean for seven females 26.57 ± 0.73, Svihla, 1932:19). Females of _P. truei_ were observed in various stages of reproduction from June through September. Ten of the 20 females of _P. truei_ taken in September were reproducing; four contained embryos and the other six were lactating. In October, only one of 17 females caught in snap traps was lactating. Lactating females were caught in live-traps as late as October 23, although most females had ceased reproduction by then. No pregnant or lactating females were observed in November. In _P. maniculatus_, puberty has been placed at 32 to 35 days for females weighing 13 grams, and in males at from 40 to 45 days, at weights of 15 to 16 grams (Jameson, 1953:45). In _P. truei_, the weight of the testes is reported to rise in March and diminish through September, with accessory organs following the same cycle (Asdell, 1964:267). Young of _P. truei_ nurse for about one month, although some litters may not be weaned until 40 days of age. Young of _P. maniculatus_ are weaned between 22 and 37 days of age (Svihla, 1932:30). Twenty-six pregnant females of _P. maniculatus_, taken in the breeding seasons of 1961-1964, contained from one to eight embryos each; the mean was 4.65 ± 1.67. Other investigators have found similar mean values in this species (Asdell, 1964:266). Thirteen females of _P. truei_ taken in the breeding seasons of 1961-1964, contained from three to six embryos each; the mean was 4.0 ± .912. Svihla (1932:25) reported litter sizes, at birth, of two to five and a mean of 2.84, in 19 litters. Other investigators have reported litter sizes of one to five with a mean of 3.4, and one to six with a mean of 3.6 (Asdell, 1964:268). Apparently _P. truei_ does not have more than six young per litter.