Heterosis, breed-of-sire and breed-of-dam effects on growth rates were estimated in a 3-yr study of the Suffolk (SU) and Shropshire (SH) breeds of sheep and their reciprocal crosses, utilizing 175 lambs reared artificially with liquid plus solid diet from 2 to 70 days and 225 lambs creep-fed a solid diet and reared with their dams to 70 days. These comprised 135 SU, 76 SH, 82 SU (sire) × SH (dam) and 107 SH × SU lambs reared to 140 days of age. Lambs by SU sires surpassed those by SH sires in growth rate from birth to 70 days (P < 0.01) and from 70 to 140 days (P < 0.05) in artificial rearing, but only from 70 to 140 days (P < 0.01) when reared by their dams. Breed of dam had no significant effect on growth rate of lambs in artificial rearing, but the lambs from SU dams surpassed those from SH dams in growth rate from birth to 70 days (P < 0.01) and from 70 to 140 days (P < 0.05) when the lambs were reared with their dams. Heterosis, as expressed by superiority of crossbreds over purebreds, was significant (P < 0.05) in growth rate from birth to 70 days and from 70 to 140 days for lambs reared artificially, but only in the latter period for lambs reared with their dams.