The present study investigated the effects of gamma-amino butyric acid (GABA)A-active drugs on the ability of light or coadministration of vasoactive intestinal peptide (VIP), peptide histidine isoleucine (PHI), and gastrin-releasing peptide (GRP) to phase delay hamster activity rhythms. Microinjection of the GABAA agonist, muscimol, significantly (p < .01) reduced the phase-delaying effect of light administered at circadian time (CT) 13.5. By contrast, microinjection of the GABAA antagonist, bicuculline, significantly (p < .01) increased the phase-delaying effect of light administered at CT 13.5. Microinjection of muscimol or bicuculline into the suprachiasmatic nucleus (SCN) produced little or no effect on circadian phase when no light pulses were provided. Coadministration of muscimol with VIP/PHI/GRP significantly (p < .01) reduced the phase-delaying effect of VIP/PHI/GRP, whereas administration of bicuculline with VIP/PHI/GRP significantly (p < .05) increased the phase-delaying effect of these peptides. These data indicate that changes in GABAA activity within the SCN can modulate the phase-delaying effects of light and VIP/PHI/GRP during the early portion of subjective night.