1. Synaptic currents and responses to acetylcholine (ACh) have been recorded at 20 degrees C from rat submandibular ganglion cells by a two micro-electrode volatage-clamp technique.2. The peak amplitude (a(p)) of excitatory synaptic currents (e.s.c.s) was linearly related to membrane potential (E(m)), with a reversal potential close to - 10 mV. E.s.c.s decayed with a bi-exponential time course, the fast phase comprising just over half the total amplitude. The time constant (tau(f)) of the fast phase was 5-9 msec, while that of the slow phase (tau(s)) was 27-45 msec. The relative amplitudes of the two components remained constant at different membrane potentials, showing that the reversal potential was the same for both.3. Both tau(f) and tau(s) increased as the cell was hyperpolarized, the ratio tau(-80)/tau(-40) being about 1.6 for both fast and slow components.4. Increasing the calcium concentration from 2.5 to 7.5 mm increased the amplitude of both components by about 40% and also prolonged the synaptic currents 30-50%, its effect being slightly greater on tau(s) than on tau(f).5. In contrast to e.s.c.s, spontaneous or potassium-evoked miniature synaptic currents (m.s.c.s) showed a simple exponential decay with a time constant (tau(m.s.c.)) very similar to tau(f). tau(m.s.c.) showed the same sensitivity to membrane potential and calcium concentration as tau(f).6. In the presence of neostigmine (10 mum) e.s.c.s were prolonged, tau(f) about 3.5-fold and tau(s) about 2.5-fold. The decay remained bi-exponential, with little change in the relative amplitude or voltage-dependence of the two components. M.s.c.s were prolonged to a lesser extent (1.5-2-fold) and the voltage dependence of tau(m.s.c.) was unaffected by neostigmine.7. Reduction of the quantal content of the e.s.c. by low calcium-high magnesium solution did not affect the time course. The relative amplitudes, and the time constants of the two components were unchanged even with a 90% reduction of a(p).8. Voltage-jump studies, in which the cell was abruptly hyperpolarized by 20-40 mV during a response to ionophoretically applied ACh, showed a relaxation pattern consisting of two distinct exponential components, whose relative amplitudes varied considerably in different cells. The two rate constants tau(f.rel) and tau(s.rel) were somewhat shorter than tau(f) and tau(s) for e.s.c.s, the difference being generally less than two-fold.9. Measurements of ACh noise also revealed two kinetic components, the time constants of which corresponded closely to tau(f) and tau(s) for e.s.c.s. On the assumption that the two components represent channels of equal conductance, the single channel conductance, gamma, was calculated to be 31+/-3 pS, similar to that of endplate channels.10. It is concluded that the two kinetic components of e.s.c.s and ACh responses probably represent two distinct classes of ACh-operated ionic channels, whose mean lifetime differs about fivefold. The two types of channel show the same ionic selectivity and their mean lifetime varies in the same way with the membrane potential. The absence of a slow component in m.s.c.s suggests that the two types of channel are spatially separate in the membrane.