1. Resting membrane potentials (RMPs), spontaneous miniature end-plate potentials (m.e.p.p.s), and evoked end-plate potentials (e.p.p.s) were recorded in phrenic nerve-hemidiaphragm preparations from rats at ages from 11 to 375 days. 2. The mean RMP increased from -64.1 +/- 1.2 mV (mean +/- S.E.) at age 11 days to -71.3 +/- 1.0 mV at age 30 days, after which there was no significant change with age. 3. The mean amplitude of m.e.p.p.s decreased from 1.088 +/- 0.070 mV at 11 days of age to 0.405 +/- 0.030 mV at 175 days of age, after which there was no significant change. 4. There was a rpaid, large increase in the frequency of m.e.p.p.s from 0.02/sec to 0.97/sec (geometric means) between 11 and 23 days of age, followed by a slower increase to 3.19/sec at 175 days of age. Subsequently there was a decrease by 2.58/sec at 375 days of age. 5. The mean of quantum content of plateau e.p.p.s elicited at a frequency of 10 Hz increased from 20.5 quanta/e.p.p. to 169.9 quanta/e.p.p. (geometric means) between 11 and 175 days of age and then decreased to 120.4 quanta/e.p.p. at 375 days of age. 6. The mean quantum content of the first e.p.p.s of trains of e.p.p.s increased from 44.2 quanta/e.p.p. to 468.8 quanta/e.p.p. (geometric means) between 11 and 175 days of age and then decreased to 358.1 quanta/e.p.p. at 375 days of age. 7. The calculated safety factor of neuromuscular transmission increased with age up to 110-175 days and subsequently decreased. 8. The change in all the above parameters occurred most rapidly in the first 6 weeks of life. The rapidity of these changes indicates that great care must be taken to ensure that control and experimental animals are adequately matched according to age, especially when rats weighing less than about 300 g are used.