An in vivo gracilis anticus preparation for microelectrode intracellular study of neuromuscular transmission was developed. The mean resting muscle fiber membrane potential was —73 mV. Resting miniature endplate potential (mepp) frequency was 2 to 4/sec in most fibers. Depolarization induced by external application of 20 millimolar K+ was slow in increasing mepp frequency. Frequency reached 100 to 150/sec rapidly with 35 millimolar K+. The potassium depolarization augmented mepp frequency was reduced up to 60% by increasing intravenous doses of diphenylhydantoin (DPH) up to 40 mg/kg with a median effective dose of 15 mg/kg. Depolarized muscle membrane potential was increased 15% after intravenously administered DPH. The DPH appears to stabilize the motor nerve terminal and muscle membrane. These actions may account for its effect in generalized myokymia and myotonia.