Eighty-eight adult female rats were fed diets containing either 0.08, 0.12, 1, 3, or 4% NaCl for at least one week prior to breeding and throughout gestation. Within 24 hours of birth, all pups were sexed based on anogenital distance, and the number of males and females were recorded. The amount of sodium chloride in mother rats' diets was inversely related to the proportion of phenotypic males in the litter. As dietary salt increased, the proportion of males decreased. These alterations in dietary NaCl affected the sex ratio without disrupting litter size or the general health of the offspring. Dietary mineral content may affect the phenotypic sex ratio through changes in the genotypic sex ratio, or alternatively, via changes in the environment in which the genes are expressed. These findings are important for basic research concerning maternal nutrition and development.