Previous trials at Newcastle have shown that plane of nutrition affects the growth performance, carcass composition and eating quality of lambs (Mustafa and Webster, 1995). It has also been found that lean tissue growth rate, a characteristic it is possible to manipulate nutritionally, influences meat quality in pigs and cattle (Whipple et al., 1990; Blanchard et al., 1995), although no similar studies have been undertaken with growing lambs. This experiment was designed to study the effect of four diets with different levels of metabolizable energy (ME) but with the same crude protein and undegradeable protein (DUP) levels on growth, carcass composition, tissue growth rate and meat quality characteristics of lambs of two breeds.