Body weights at birth, weaning, yearling and 2 years of age were studied on 710 lambs born from 1956 to 1967 and representing Barki, Merino and their F1 and back-cross. Body weights of the lambs of the two cross-bred groups were intermediate between those of the two pure-breds at birth and exceeded them at weaning. Merino lambs were 0.61 kg heavier than Barki lambs at birth and maintained that advantage throughout the study. Age of dam, type of birth, sex, and year of birth were significant sources of variation on the weights studied. The effect of age of dam and type of birth declined after weaning while that of sex increased. © 1974, Cambridge University Press. All rights reserved.