Cortical perfusion with GABA agonists and antagonists modulates the spontaneous release of cortical acetylcholine and GABA in freely moving rats. Twenty-four hours after implantation of a dialysis fibre, cerebral cortex spontaneously released acetylcholine (3.8 ± 0.2 pmol/10 min) and GABA (6.6 ± 0.4 pmol/10 min) at a stable rate. Local administration of GABA (1 or 5 mm) or the GABAA agonist muscimol (25 or 50 μm) had no effect on the spontaneous release of acetylcholine. However, bicuculline (1–25 μm), a GABAA antagonist, added to the dialysis perfusate, elicited a concentration-dependent increase of acetylcholine release to approximately double that of control. This effect of bicuculline (25 μm) was completely prevented by coperfusion with muscimol (50 μm). Local administration of the GABAB receptor agonist baclofen (10 or 50 μm) elicited a concentration-dependent increase in spontaneous acetylcholine release with a maximal increase of about 60%. Intracortical administration of baclofen also decreased the spontaneous release of GABA. The GABAB receptor antagonist CGP 35348 (1 mm), administered alone for 20 min through the dialysis fibre, was without effect on spontaneous acetylcholine release; however, it completely blocked both the baclofen-induced increase in acetylcholine release and the decrease in GABA release. These results suggest that cortically released GABA exerts a tonic influence on cholinergic activity.