In this study, we will test the following hypothesis: A high-leucine essential amino acid mixture (dose of 6.7 g) will stimulate protein anabolism to a greater extent than a standard balanced mixture of total (essential and non-essential) amino acids in CF pediatric subjects. The principal endpoints will be the extent of stimulation of protein synthesis rate and the reduction in endogenous protein breakdown. The current project will provide information that will enable us to better understand the underlying metabolic mechanisms that regulate protein metabolism in pediatric subjects with CF.