Dual type of facilitation at the neuromuscular junction of the frog

End-plate potentials were recorded intracellularly at the frog neuromuscular junction. Facilitation produced by conditioning shocks applied to the motor nerve was studied in two different ways: using either low-frequency trains in Mg-blocked preparations or using brief high-frequency trains followed by a test shock of variable latency in D-tubocurarine-blocked preparations. In both cases, two distinctly different types of motor end-plates were observed. The first type showed a low quantum content and a marked facilitation without depression, while the second type showed a high quantum content, a poor facilitation, and a marked depression. The physiological significance of these findings is discussed.