Excitatory postsynaptic currents (EPSCs), miniature excitatory postsynaptic currents (MEPSCs), and acetylcholine-induced current fluctuations (noise) have been studied in voltage-clamped bullfrog parasympathetic ganglion cells of the atrial septum. EPSCs were also recorded from voltage-clamped sympathetic B cells and, in general, it was found that the basic properties of the EPSCs are similar in both parasympathetic and sympathetic B cells. For parasympathetic cells, the EPSC reached a peak amplitude of several nanoamperes within 3 msec and decayed exponentially. For 31 cells voltage-clamped to -50 mV (22 to 23 degrees C), peak amplitude was -4.3 +/- 1.4 nA (mean +/- SD) and the decay time constant, tau, was 5.6 +/- 1.0 msec. tau was independent of EPSC amplitude at a set voltage but increased with hyperpolarization, the coefficient of voltage dependence being -0.0070 +/- 0.0025 mV-1 in 13 cells (21 to 23 degrees C). The EPSC amplitude-voltage relationship was linear between -30 and -90 mV. The reversal potential, determined by interpolation, was -4.0 +/- 6.7 mV (n = 11). The EPSC tau had a Q10 equal to 2.9. Blocking the acetylcholinesterase with methane sulfonyl fluoride (MSF) pretreatment prolonged EPSC decay but decreased EPSC amplitude. In addition, EPSC decay after MSF treatment deviated from a single exponential function. MEPSCs exhibited decay characteristics very similar to those of EPSCs recorded at the same voltage and temperature. Acetylcholine-induced current fluctuations were well described by a single Lorentzian function with the estimated mean channel open time (tau noise) very similar to the EPSC decay time constant.(ABSTRACT TRUNCATED AT 250 WORDS)