Skinned, single-fiber preparations from the quadriceps or gastrocneumius muscles of four ambulatory male children with Duchenne dystrophy were tested for theri ability to generate tension and to regulate CA++. To determine the intrinsic strength (P0) of the contractile material, the maximum Ca++ -activated tensions were normalized to the fiber diameters. Sixty-four percent of the Duchenne fibers had P0 values below 1.0 kg per square centimeter--the lowest value observed in control muscle--and the average P0 values of fibers from each Duchenne biopsy were significantly (p less than 0.01) below the average P0 values for control muscle fibers and for muscle fibers obtained from one obligatory carrier of the Duchenne gene. The low tensions in the Duchenne muscle fibers could not be ascribed to altered Ca++ regulation or to substrate sensitivity of the contractile proteins in the fibers, since these were normal. However, ultrastructural abnormalities of the myofilaments, which might reduce the ability of the contractile system to develop tension, were observed. Furthermore, Ca++ regulation by the sarcoplasmic reticulum (SR) was impaired in most of those muscle fibers, from both carriers and Duchenne patients, that did develop normal tension. These results suggest that in Duchenne muscle a functional disorder in the SR may precede loss of the ability of the contractile proteins to generate tension. However, since muscle fibers from Duchenne-gene carriers developed significantly greater tensions than fibers from Duchenne-patients, while yet having similar defects in Ca++ regulation, the SR disorder may not be exclusively responsible for abnormal contractile protein function.