1. Experiments were done on the neuromuscular junction of the frog to study facilitation of the end-plate potential (e.p.p.) produced by one or more conditioning shocks applied to the motor nerve. Neuromuscular transmission was blocked with Mg (+)-tubocurarine chloride.2. In Mg-blocked preparations, facilitation increased with increasing Mg concentrations in the range 14-34 mM. The increase in facilitation was related specifically to the decrease in mean quantum content of the e.p.p., and was most marked with quantum contents less than 10.3. Facilitation in curarized preparations was similar in magnitude to that in preparations blocked with the lower Mg concentrations. Interaction between facilitation and depression in curarized preparations was consistent with the idea that facilitation was due to an increase in probability of transmitter release and depression to depletion of transmitter.