Analysis of the photic induction of c-Fos immunoreactivity (-ir) within the suprachiasmatic nucleus (SCN) has proven to be a powerful tool with which to study the neurochemical mechanisms involved in phase shifting the circadian clock. Some systemically administered GABAergic drugs inhibit light-induced phase shifts and c-Fos-ir, whereas others inhibit light-induced phase shifts without affecting c-Fos-ir. More recently, we have found that injection of GABAergic drugs directly into the SCN region can have dramatically different effects on light-induced phase shifts than following their systemic administration. The present study investigated the effects of GABA A and GABA B agonists and antagonists injected into the SCN region on c-Fos-ir within the SCN. Microinjection of either a GABA A agonist, muscimol, or a GABA B agonist, baclofen, into the SCN region significantly reduced lightinduced c-Fos-ir within the SCN when administered before light exposure at circadian time (CT) 13.5 or CT 19. In contrast, microinjection of a GABA A antagonist, bicuculline, but not a GABA B antagonist, CGP-35348, into the SCN region increased light-induced c-Fos-ir within the SCN when administered before light exposure at CT 13.5 or CT 19. These data indicate that GABAergic agonists and antagonists injected directly into the SCN region alter light-induced Fos-ir in a manner similar to their effects on light-induced phase shifts. Comparison of these data with previous studies examining the effects of systemically administered GABAergic drugs suggests that GABA B -active drugs have similar effects whether given systemically or within the SCN, but that GABA A -active drugs have more complex effects on c-fos induction and have multiple sites of action.