Capillary electrophoresis (CE) with postcolumn laser-induced fluorescence detection (LIF) was used to analyze single skeletal muscle fibers from young and old rats. Due to selective labeling of mitochondria with 10-N-nonyl acridine orange, the zeptomole (10(-21) mole) sensitivity, and the high separation power, three properties of individual mitochondrial particles were revealed: the number, the distributions of cardiolipin, and their electrophoretic mobilities. Type I fibers had more mitochondrial particles and cardiolipin per particle than did type IIb fibers from rats of similar age. Individual fibers of the same fiber type from young rats contained more mitochondrial particles and cardiolipin per particle than did fibers from old rats. There were fiber type-specific and age-specific differences in the electrophoretic mobility of individual mitochondrial particles. The CE-LIF results of individual mitochondrial particles are the first of their kind in that they reveal fiber type-specific and age-specific differences that are not obviously noticed in bulk measurements of heterogeneous tissues.