In addition to ionotropic effects, glutamate and acetylcholine have metabotropic modulatory effects on many neurons. Here we show that in the stomatogastric ganglion of the lobster, glutamate, one of the main ionotropic neurotransmitters, modulates the excitability of gastric mill neurons. The neurons in this well-studied system, produce rhythmic output to a subset of lobster foregut muscles. Recently, metabotropic glutamate receptor (mGluR) agonists were suggested as modulators of the rhythmic output, in addition to the previously described muscarinic modulation by acetylcholine. However, the cellular mechanisms responsible for these effects on the pattern are not known. Using intracellular recording methods and calcium imaging, we show that glutamate has an excitatory effect on specific neurons in the STG, which is mediated by mGluRs. The response to mGluR type I agonists application is transient oscillations in the system, probably due to network interactions. We show that the excitatory effect is phospholipase-C and IP₃ sensitive, as well as, G-protein dependent. The G-protein dependence was demonstrated by GDPS and GTPS injection into identified neurons. The depolarization and oscillation were accompanied by an increase of intracellular Ca²⁺ levels and correlated Ca²⁺ oscillations. By using cyclopiazonic acid, an ER Ca²⁺ uptake inhibitor, we show that some internal calcium release may augment the response but is not crucial for its production. Interestingly, although Ca²⁺concentration increase is typically associated with the phosphoinositide pathway, in the lobster, Ca²⁺ concentration increase, either voltage dependent or independent, cannot account for the observed depolarization.