The ontogenic change in properties of muscle membrane and neuromuscular junctions in fast (m. extensor propatagialis longus; EPL and m. pectoralis major; PM) and slow (m. anterior latissimus dorsi; ALD) muscle fibers of normal and inherited dystrophic chickens were investigated. In normal and dystrophic EPL and ALD, the membrane potential increased between 3 and 6 weeks ex ovo, and remained much the same value until 30 weeks ex ovo. There was no apparent difference in membrane potential between both strains. In ALDs of both strains, the threshold depolarization for the spike generation remained the same until 10 weeks ex ovo but thereafter the spike became abortive in shape. In EPLs of both strains, the threshold potential shifted to a hyperpolarized level following the increase in membrane potential. In PM and EPL of both strains, length constant, time constant of the membrane, membrane resistance and cell diameter increased, whereas input resistance decreased. However, the above changes in the various membrane parameters appeared to be more significant in dystrophic chickens than in normal ones at 3-6 weeks ex ovo. A part of these changes in the various parameters could not be explained by increased cell diameter alone. Changes in the membrane property during the progress of aging in dystrophic chickens appeared simultaneously in both muscles. The properties of m.e.p.p.s. and e.p.p.s in ALD and EPL in the normal chicken showed specific patterns such as slow and fast muscles, respectively. However, in ALD and EPL, no difference in the properties of acetylcholine receptor and acetylcholine release from nerve terminals between both strains was observed. These results suggest that pathological changes in the muscle cell are evident both in the membrane and the contractile properties but not in the neuromuscular junction.