1. End-plate potentials (e.p.p.s) were recorded intracellularly from the isolated phrenic/diaphragm preparation of the rat during the first few weeks of life.2. Most e.p.p.s at birth were complex and resulted from the summation of two to four units, which could be separated by their different latencies and thresholds to stimulation of the phrenic nerve.3. The e.p.p.s became simpler during the second week of the rats' life, and by 16-18 days old the e.p.p.s consisted of single units, and resembled the e.p.p.s of adult rat muscle.4. It is proposed that the units of the e.p.p. resulted from the stimulation of separate nerve axons and that all but one of the synapses on each muscle fibre were lost during the second week of life.