This study was designed to define the age-related changes that occur in the F1 cross of the male Fischer 344 and brown Norway rats and to determine if these findings were associated with electrophysiologic abnormalities indicative of motor neuron loss. Contractility, morphologic, and histochemical studies were performed on the tibialis anterior muscles (TA) from 25 male rats at ages 6, 18 and 30-32 months. Tibialis anterior weight was 17% greater in the 18-mo vs 6-mo-old animals, but at age 32 months mean TA weight was 20% less than at 18 months. Other changes at 32 months included a 12% decrease in specific tension and reduced contractile/relaxation velocities of isometric twitches and maximal tetanic tension; findings associated with a 40% decrease in type IIb fiber cross-sectional area. Electrophysiologic studies on 15 rats revealed prolonged H-reflex latencies at 18 and 32 months. Needle electromyography demonstrated abnormal spontaneous activity consistent with peripheral axonal, not motor neuron loss. These findings demonstrate age-related changes in muscle mass and strength that are associated with changes in the peripheral nervous system. These findings are consistent with previous work in homozygous, inbred strains and help to establish the F1 cross of the Fischer 344 and brown Norway strains as a potentially useful rodent model in gerontologic studies of the neuromuscular system.