Seventy-eight Beulah (BH) and 78 Welsh Mountain (WM) wether lambs (mature live weights (MLW) 60 and 70 kg respectively), were used to investigate genotypic differences in performance and compensatory growth during the post-weaning period. After an initial slaughter of 12 of each breed, a complete straw-based pelleted diet (122 g crude protein and 10·4 Mj metabolizable energy per kg dry matter (DM)) was offered ad libitum to a further 29 of each which were slaughtered at a range of live weights between proportionately 0·35 andl -00 of ML W. As well as these groups undergoing normal growth (NG), the remaining lambs were given the same diet at a level sufficient to maintain constant live weight (LW) for 3 months. Eight of each breed were then slaughtered and the remaining 29 of each offered the diet ad libitum for a period of post-restricted growth (PRG); these 29 were also slaughtered between 0·35 and 1·00 of MLW. During the periods of ad libitum food supply, voluntary food intake was greater in BH than WM lambs (BH: 38·3, WM: 35·3 g DM per kg LW, P < 0·01) and during PRG as compared with NG although this result was due to a response only in WM lambs (NG - BH: 38·0, PRG-BH: 38·5, NG - WM: 33·3, PRG - WM: 37·2 g DM per kg LW, breed × pattern of food supply interaction, P < 0·05). Daily live-weight gain (LWG) was greater during PRG than NG in both breeds (NG: 3·15, PRG: 4·80 g/kg LWper day, P < 0·05). This effect was greatest in the first 8 weeks of growth but persisted into the period 16 weeks to maturity. WM lambs had overall slightly leaner carcasses and a lower proportion of protein in the fleece-free empty body than BH when compared at the same proportion of MLW; particularly during PRG. The protein in the carcass-weight gain (CWG) was greater in WM lambs during PRG than during NG (NG: 98, PRG: 123 g/kg, P < 0·05), but did not differ significantly in BH (NG: 106, PRG: 118 g/kg). The proportion of fat in CWG was concomitantly smaller during PRG. There are thus breed differences in the performance and carcass composition of lambs during compensatory growth. Genotypes to be finished by continuous feeding or a regimen incorporating food restriction should be carefully chosen for date offinishing in relation to desired carcass composition.