The size of miniature end-plate potentials (m.e.p.p.s) and miniature end-plate currents (m.e.p.c.s) at frog neuromuscular junctions was increased by a factor of two or more following treatment with norepinephrine, epinephrine, a cAMP derivative, insulin or ACTH. The increase in size was reversible. These agents do not appear to alter the ACh noise spectrum or the reversal potential of the end-plate. They do not alter the binding of alpha-bungarotoxin. Therefore they may increase the quantity of acetylcholine (ACh) in a quantum.