Litter or clutch size is an important component of an organism's total reproductive effort. Because of its importance, the evolution of litter or clutch size has received considerable theoretical attention (e.g., Brockelman, 1975; Cody, 1966; Lack, 1948; Mountford, 1968; Ricklefs, 1970; and Smith and Fretwell, 1974). Most authors point out that optimal clutch sizes are the products of diverse selective pressures, including levels of resource abundance, the intensity of intraand interspecific competition, predation pressure, and other sources of mortality. In the face of these diverse pressures, organisms can produce clutches containing a relatively large number of offspring each of small size or clutches containing a smaller number of relatively large offspring. The balance that is struck between offspring size and number often depends on the relative importance of the selective pressures determining both parental and offspring fitness. In highly competitive environments, for example, selection will favor increased parental investment per offspring and decreased litter or clutch size whereas in environments in which predation and/or climatic changes cause heavy mortality, reduced parental investment per offspring and increased litter or clutch size will be selectively favored (Brockelman, 1975). Behavioral complexity can also influence the evolution of clutch or litter size. In vertebrates, large clutches of small hatchlings often occur in fish, amphibians, and reptiles that lack parental care whereas small clutches or litters of larger young occur in birds and mammals in which parental care is usually much more highly developed. In this paper we examine experimentally some of the factors that may determine optimal litter size in the common North American forest-dwelling cricetid rodent Peromyscus leucopus. Results of a number of studies (summarized in Fleming (1970) and Hill (1972)) indicate that average litter size in this species, whose adults weigh 22-25 g, ranges from 4.1 to 5.5. Part of this variation is geographic (Smith and McGinnis, 1968) and part is age (parity)-related (Drickamer and Vestal, 1973). In laboratory colonies of P. leucopus noveboracensis, the subspecies with which we worked, litter size averages 4.7 (Drickamer and Vestal, 1973; Hill, 1972). The basic question we set out to answer was: Is a litter size of 4.7 (or its closest integer value, 5) most fit? Does it produce the greatest number of surviving and reproducing offspring, as Lack (1948), but not Mountford (1968), would predict? In attempting to answer this question, we have examined the effect of litter size and female parity on the following parameters: (1) birth weight, (2) survivorship and growth rates of young during their first four weeks of life, (3) weight at weaning, and (4) the ability of weanlings to survive exposure to acute stress.