Excitatory synaptic currents (EPSCs) were recorded extracelularlly from synaptic spots on crayfish opener muscles. The decay of facilitation after a first pulse was measured by a second pulse given at increasing intervals; the duration of facilitation TF was the interval after which the second EPSC had 1.1 times the amplitude of the first one. Increasing [Mg]0 in the range from 0.5-12.5 mM at low [Ca]0 (1.7-4.5 mM) led to a monotonic prolongation of facilitation. TF showed a S-shaped dependence on [Mg]0, rising very steeply at 2-5 mM [Mg]0. At higher [Ca]0, and also at half normal [Na]0, an increase of [Mg]0 did not affect the decay of facilitation appreciably. As shown before, the decay of facilitation is due to two Cai-removal processes, R1 and R2. Mg0 inhibits only the R1 process, which is also inhibited by high [Ca]0, is dependent on normal [Na]0 and has the characteristics of a Cai in equilibrium with Na0 exchange. As one possible mechanism, competition of Mg0 with Na0 at the extracellular loading site of the exchange is discussed quantitatively.