1. In two experiments with growing and fattening lambs sodium and calcium salts of volatile fatty acids (VFA) were added to a basal diet of hay and concentrate. In Expt I the lambs were approaching maturity, and the utilization of salts of acetic, propionic and butyric acids was studied. In Expt 2 the animals were younger; the experimental groups received acetate or propionate, and half of them were implanted with hexoestrol. The chemical composition of the carcass was assessed with the loin as a representative joint. The influence of the VFA salts on the digestibility of the basal ration was also investigated. 2. In both experiments the lambs receiving supplements of VFA salts grew faster, and their empty body weights and carcass weights were significantly greater than in lambs receiving the basal diet. 3. The energy derived from acetate and propionate was utilized more efficiently to promote carcass gains than the calculated metabolizable energy above maintenance of the basal ration. There were no differences in the utilization of energy from the different VFA, except in Expt I in which butyrate was utilized somewhat less efficiently than acetate and propionate. 4. Hexoes-trol implantation resulted in faster growth, and highly significantly greater empty body weights and carcass weights. There was no evidence of an interaction between the VFA and hexoestrol treatments. 5. There were no differences between the VFA treatments in the composition of the carcass gains as judged by the composition of the loins, except in Expt I in which lambs receiving acetate tended to be the fattest. The coefficient of variation in loin fat percentages was large in both experiments, but, in Expt I in which both ewe and wether lambs were used, the coefficient of variation among the wether lambs was four times that of the ewe lambs. 6. Additions of VFA salts to the diet resulted in a significant increase in the excretion of ash.