Populations of the old-field mouse (Peromyscus polionotus) were snap-trapped in fields of the Savannah River Project, South Carolina, from 1955 through 1961. Litter size of 172 pregnant females was 3.13 (SD = 0.86). Total length was chosen as the statistic best representative of body size. The 126-135 mm mature females were pregnant more frequently (34%) and had larger embryo counts (3.47, 1SE = 0.17) than the 98-115 mm mature females (21% and 2.95 ? 0.10). Breeding peaks occurred in spring and fall for all size classes with 50% less activity in the summer and virtually none in the winter. Summer litters were significantly smaller (2.76, 1SE = 0.19) than litters of spring and fall (3.48 ? 0.20). The observed effect of body size on natality could cause a 12% recruitment difference between natural populations. At the Atomic Energy Commission's Savannah River Project (SRP) in South Carolina, several species of animals have been intensively studied by University of Georgia biologists as part of a broad program of investigations of the old-field ecosystem. Of the species the old-field mouse is the only abundant rodent of the SRP old-field ecosystem during early stages of plant succession. Recent publications by the University group have dealt with population densities of P. polionotus, population interactions, and the relationship of the species to energy flow in the ecosystem (Caldwell, 1964; Davenport, 1964; Gentry, 1964; and Odum, Connell, and Davenport, 1963). The present paper reports on some effects of the environment on reprcduction of P. polionotus. Reproductive performance of a laboratory colony of old-field mice derived from the SRP population has been described by Williams, Golley, and Carmon (1965). Upon abandonment from agriculture in 1951, fields of SRP reverted to herbs and forbs such as Aristida sp., Leptilon canadense, Haplopappus divaricatus, Heterotheca subaxillaris, and Erigeron sp. A second stage of succession was typically characterized by perennial Andropogon virginicus and A. ternarius. However, certain fields did not follow this sere (Menhinick, 1963) but developed almost pure stands of Lespedeza cuneata and L. striata. These two species have persisted since the pre-1951 cultivation era; they annually produce abundant seed crops and have invaded other old-field areas. Data for this paper were gathered from 1955 to 1961. During these years the three categories of vegetation just described were each sampled in the trapping studies. Succession in some fields proceeded more slowly than in others and thus, even though several years passed since succession had begun, we were able simultaneously to sample populations from both early and late seral stages. 1 This work was supported by contract AT (30-1) -310 between the Atomic Energy Commission and the University of Georgia.