Calcium uptake by mitochondria isolated from skin fibroblasts of patients with cystic fibrosis and controls was studied in the presence and absence of inhibitors. Since mitochondrial calcium accumulation may be supported by ATP hydrolysis or respiration, inhibitors of each were used to characterize the basis of previously described alterations in calcium uptake by mitochondria from patients with cystic fibrosis. Calcium uptake measurements under the influence of oligomycin and antimycin A suggest that the increased calcium uptake by mitochondria from patients with cystic fibrosis is related to altered respiratory system activity. Binding constants of calcium to the carrier system in mitochondria were not different between genotypes.