Although there is a general consensus that housing conditions affect the well-being of laboratory animals, the ideal cage size and density for housing laboratory rodents has not been established. The authors investigated the effects of cage size and cage density on growth, organ development, metabolic profile, and hemogram in juvenile Sprague-Dawley rats. Larger cages and increased cage density were associated with depressions in body weight and in the weights of several organs. In general, increasing group size and density correlated more strongly with detrimental effects on the growth of females than males, although hemogram values indicated that males are more prone to emotional stress and immune suppression than females in response to increasing group size and crowding.