The ACh [acetylcholine] output from the sympathetic pregangl?onic nerve terminals of the toad was measured by the longitudinally semi- or multisectioned lung and was analyzed according to electrophysiologic and submicroscopic observations of the ganglionic synapses. The postganglionic B and C responses induced at a rate of 1/sec. were well maintained for over 2 hr., while those elicited at a rate over 5/sec. showed an exponential lowering in their height. Measurement of the outward diffusion of ACh in the ganglia indicated that more than 95% of ACh moved out of the ganglia into the bathing solution in 30 min. With 1/sec. preganglionic stimulation the eserinized ganglion released on the average of 1.6 x 10-16 g of ACh per volley for a single ganglion cell and the TEPP [tetraethylpyrophosphate]-treated ganglion, 2.6 x 10-16 g. with 5/sec, 10/sec, and 100/sec. preganglionic stimulation, the eserinized ganglion released, on the average, 0.9 x 10-26 g, 0.6 x 10-16 [OMEGA] and 0.19 x 10-16 g, respectively, of ACh per volley for a single ganglion cell. The average quantum content of an EPSP [excitatory postsynaptic potentials] was 129 for sB neurons and it was 79 for sC neurons. It was estimated by electron microscopic observation that average subsynaptic membrane areas of single sB and sC neurons were 385 [mu]2 and 102 [mu]2, and that approximately 55 synaptic knobs terminated on a sB neuron and 15 knobs on a sC neuron. The maximum shunting conductance of the subsynaptic membrane during EPSP and miniature EPSP was 0.85 x 10-6 [OMEGA] and 6.6 x 10-9 [OMEGA] for sB neurons and 0.2 x 10-6 [OMEGA] and 1.7 x 10-9 [OMEGA] for sC neurons, respectively. The specific resistance of the activated subsynaptic membrane of sB neurons appeared to be 4.6 [OMEGA] cm2 and that of sC neurons, 5.1 [OMEGA] cm2. A single knob on sB neurons liberated approximately 2 quanta of ACh per impulse and a single knob on sC neurons about 5 quanta. In a sB synaptic cleft the concentration of liberated ACh per impulse may reach about 2.3 x 10-4 moles/liter and in a sC synaptic cleft it may reach as high as 8.4 x 10-4 moles/liter in the presence of an antichollnesterase.