The present study tested the hypotheses that spontaneous gamma-aminobutyric acid (GABA) efflux in anterior rat striatum is 1) independent of intra- and extracellular calcium; and 2) is physiologically relevant. Extracellular dopamine (DA) and GABA were sampled from striatum of awake, freely moving rats using in vivo microdialysis. Although dialysate concentrations of DA were 2 to 3 times greater than GABA and were decreased by at least 70% by removal of calcium, GABA was unaffected even in the presence of EGTA or the intracellular calcium chelator APTRA-AM. Functional significance of this non-exocytotic pool of GABA was tested by injecting 3-mercaptopropionic acid (3-MPA), an inhibitor of GABA synthesis, into the striatum via a guide cannula sidled alongside a microdialysis probe and measuring subsequent effects on behavior and perfusate concentrations of GABA. Results show that 3-MPA increases gnawing behavior suggesting that basal, non-exocytotic GABA overflow normally functions to suppress gnawing.