This study was conducted to measure neuromuscular transmission in newborn infants. Age-dependent differences in neuromuscular transmission and the effect of nitrous oxide anesthesia upon neuromuscular function were assessed in pediatric surgical patients following induction of anesthesia with methohexital by the use of the frequency sweep electromyogram (FS-EMG). Children older than 12 weeks' chronologic age usually had FS-EMG responses similar to those of adults, whereas infants less than 12 weeks old had significantly less pronounced FS-EMG responses at high stimulation frequencies (>50 Hz). Administration of 70 per cent nitrous oxide induced 11-38 per cent reductions in the amplitudes of the FS-EMG responses at all frequencies of stimulation in the younger group. A positive correlation was found between inability to sustain a tetanic contracture (FS-EMG fade) in the 50-100-Hz region of stimulation and percentage depression of the FS-EMG response induced by nitrous oxide.