Spontaneous miniature end-plate currents (m.e.p.c.s.) were recorded in rat diaphragm at 7, 22 and 37 degrees C at -80 mV. The onset rate, measured as 20-80% rise time, was sensitive to temperature with activation energy 14 kcal mol-1 deg-1, and was not sensitive to membrane voltage between -60 and -130 mV. 2. The rise time recorded by external electrodes was 144 microseconds at 37 degrees C (6) and was similar to that found by internal electrodes. 3. The fall time was temperature-sensitive with activation 18 kcal, and was prolonged when the end-plate was hyperpolarized. 4. With acetylcholine (10 microM) the current increased to a peak and then fell within 30 s to a value which declined slowly. From fluctuation analysis the channel open time of 237 microseconds (7) at 37 degrees C was estimated. External recording gave comparable values (4). Comparison of the initial estimates with those obtained after 3-6 min of continued application showed no consistent change. The channel conductance was 26 pS at 37 degrees C. 5. The time constant of m.e.p.c. decay was consistently longer than the channel open time obtained from noise analysis. 6. With carbachol (40 microM) the current increased to a peak and then declined to a steady value. Fluctuation analysis by internal and external recording gave an increase of 5% in root mean square current with channel open time of 83 microseconds (6) at 37 degrees C, and channel conductance 17 pS.
