Histologically, there is a continuous gradation in myofibrillar content between normal and severely affected muscles. Myofibrils in affected fibers are scanty and small in cross section. No other morphologic or histochemical abnormalities have been detected. The cause of the condition remains obscure. Newborn piglets of hybrid stock tested for defects of neuromuscular transmission by stimulation electromyography reveal that congenital myofibrillar hypoplasia is not primarily a myasthenia-like syndrome, but that either excitation-contraction coupling or the contractile mechanism itself is primarily affected.