beta-Endorphin and a C-terminal analogue have been shown to decrease muscle fatigue and increase glucose uptake in muscles of normal mice. In order to provide evidence whether these peptides might be useful in muscle-wasting conditions and whether the two actions of the peptides are interdependent, the effect of beta-endorphin on muscle fatigue and glucose uptake was studied using isolated hemidiaphragm preparations of dystrophic mice as well as normal mice. Muscle contractions were elicited by high-frequency stimulation of the phrenic nerve. Glucose uptake was measured using (nonmetabolizable) 2-deoxy-D-[1-(3)H]glucose. beta-Endorphin and the C-terminal analogue reduced fatigue in normal muscles of males but not females. Insulin had no effect in either sex. The peptides increased 2-deoxyglucose uptake in contracting and noncontracting muscles of normal males and females. beta-Endorphin reduced fatigue and increased deoxyglucose uptake in dystrophic muscles. The effect on fatigue was not due to increased glucose uptake, as the energy substrate present was pyruvate. Nerve stimulation released beta-endorphin immunoreactivity from intramuscular nerves of dystrophic mice. It is hypothesized that beta-endorphin released from motor nerves as well as from the pituitary could be responsible for improving muscle function during exercise. beta-Endorphin or analogues could have therapeutic use in muscle-wasting disease.