Quantal endplate currents (qEPCs) were recorded extracellularly by a macropatch electrode from excised diaphragms of mice. During the first 3 days after birth, the mean rise time t(r) was 0.5 ms (0.1-0.9 peak, 20 degrees C). Double-exponential, amplitude-weighted fits of the decay discerned almost equally abundant components tau(1)' approximately equal to 6 ms and tau(2)' approximately equal to 9 ms. Beginning on day 3, on days 4 and 5 after birth both t(r) and the tau' dropped. Further decreasing slowly, adult values were reached at day 8, with t(r) approximately equal to 0.3 ms, tau(1)' approximately equal to 2 ms and a very weak tau(2)' approximately equal to 6 ms component. When compared to the kinetics of fetal channels, the tau(1)' and tau(2)' of up to 3 day qEPCs could correspond to the short and long splice variants of the fetal channel type. The tau(1)' of adult muscles of 2 ms agrees well with the burst durations of adult channels while a weak longer tau(2)' component may represent 'extrasynaptic' channels. The long t(r) of very young mice may correspond to the relatively slow rise of channel currents elicited by ACh pulses in mouse myoballs.