L-Cysteinesulfinic acid is a potent agonist at several rat metabotropic glutamate receptors (mGluRs) with pEC50s of 3.92±0.03, 4.6±0.2, 3.9±0.2, 2.7±0.2, 4.0±0.2, and 3.94±0.08 for mGluR1, mGluR5, mGluR2, mGluR4, mGluR6, and mGluR8, respectively. L-Cysteinesulfinic acid is an endogenous agonist of a metabotropic receptor coupled to stimulation of phospholipase D (PLD) activity. L-CSA is an endogenous agonist of the PLD-coupled metabotropic excitatory amino acids (EAA) receptor. L-CSA selectively activates the PLD-coupled receptor. 1 mM L-CSA induces a significant increase in PLD activity in hippocampal slices, whereas 1 mM concentrations of L-glutamate, L-aspartate, and L-HCA are without effect. L-CSA elicits a dose-dependent increase in PLD activity in rat hippocampal slices in the presence of iGluR antagonists, with an approximate EC50 of 500 uM. The PLD response induced by 1 mM L-CSA is not significantly decreased in the presence of 1 uM tetrodotoxin, suggesting that this response is not dependent upon L-CSA-induced increasesin cell firing.