Growth and development of heart and lungs were measured from day 15 of incubation to 25 days after hatch in lines of chickens that had undergone long-term selection for high (HAS) or low (LAS) antibody titers to sheep erythrocytes. A correlated response to this selection was heavier 28-day body weights for LAS than HAS chickens. In this experiment body weights were heavier in line LAS than HAS from 15 days of incubation to 5 days after hatch and again at 25 days after hatch. Absorption of residual yolk was greater in HAS than LAS chicks. Although there were no differences between lines for absolute heart weights, lungs were heavier in line LAS than HAS at all ages except 20 days after hatch. Relative to body weight, both heart and lung weights declined with age, however, the pattern of decline differed. For this criterium, hearts were larger in line HAS than LAS to 5 days after hatch with no difference thereafter; for lungs lines were similar until 5 days after hatch after which they were larger in LAS than HAS chicks. In all but one case (HAS at hatch) the left:right relationship for lung weights exhibited fluctuating asymmetry with the left minus right character difference having a normal distribution and mean zero. The degree of fluctuating asymmetry, however, varied with age.