Elevation of brain magnesium enhances cognitive capacity. Chronic lead (Pb) exposure has been implicated in the impairment of cognition. Whether elevation of brain magnesium rescues the impairment of memory by chronic Pb exposure is still unclear. Here we investigated the effect and possible mechanisms of chronic dietary intake of magnesium-L-threonate (MgT) on impaired spatial memory by chronic developmental Pb exposure in rats. We found that MgT rescued impaired spatial memory ability in Pb-exposed rats. This positive effect is relative to increasing dendritic spine density and the expression of NR2A and GluR1.