Growth allometry was examined over the range 6 to 112 days of age in male and female mice from lines selected for low (L) or high (H) plasma concentrations of insulin-like growth factor-I (IGF-I). Plasma IGF-I concentrations were greater in the H line than in L line mice from 28 days of age. H line mice also had greater liveweights and weights of the heart, kidneys, pancreas, lungs, liver, brain and testes from 21-28 days of age. Changes in weights of these organs reflected the general pattern of body growth, there being no consistent effects of selection line on allometric growth coefficients. Ovarian weights were not different between the lines. For muscle weights, (gastrocnemius, quadriceps femoris), the weights or lengths of bones (tibia, femur) and the nose-anus and anus-tail lengths, allometric growth coefficients were generally higher (P less than .05) in L line females than in L line males or H line mice of either sex. The allometric growth coefficient for spleen was significantly (P less than .05) greater in H line mice than in L line mice with the result that spleen weights were 30% higher in H line mice from 28 days of age. This is consistent with results from mice treated with, or transgenic for, IGF-I and suggests a specific effect of this hormone on growth of the spleen. Thymus weights were also greater in H line than in L line mice and developmental patterns of thymus weight closely paralleled those of circulating IGF-I.