At the frog neuromuscular junction, calcium-induced calcium release facilitates EPP amplitudes in conditions of low Ca2+ and high-Mg2+ (Narita et al, 2000), whereas there have been no reports of a role for IP3 receptors in short term plasticity at this synapse. Here we investigated the role of these receptors in short-term plasticity recorded in Ringer containing a physiological concentration of calcium ion. Experiments were performed on isolated nerve muscle preparations from young frogs of either sex weighing 70-90 g. The animals were sacrificed by double pithing according to local animal care guidelines. Intracellular recordings of end-plate potentials (EPPs) in response to suprathreshold nerve stimulation were obtained from cutaneous pectoris or sartorius muscle fibers in normal amphibian Ringer containing d-tubocurarine (6 µM) to block muscle contractions. Muscles were treated with either ryanodine (10 µM), 2-APB (10 µM) or U-73122 (5 µM) to block ryanodine receptores and calcium-induced calcium release, IP3 receptors or phopholipase C, respectively. Control recordings were obtained in the corresponding drug vehicles (0,1% ethanol, 0.002% methanol, 0.05% DMSO, respectively). Treatment with ryanodine caused a decrease in paired-pulse facilitation for intervals < 20 ms and a marked decrease in the extent of tetanic depression. Treatment with 2-APB caused a reduction in EPP amplitude, increased paired-pulse facilitation for intervals < 20 ms, and significantly reduced the extent of tetanic depression. Treatment with U-73122 caused a decrease in EPP amplitude, decreased paired pulse facilitation for intervals < 20 ms, and increased the extent of tetanic depression. We conclude that ryanodine receptors, IP3 receptors, and phopholipase C contribute to short-term plasticity at the frog neuromuscular junction