Recent work in our laboratory has shown that sodium pentobarbital injections can induce phase-dependent phase shifts of the circadian rhythm of locomotor activity with the maximum advance at circadian time (CT) 8 and the maximum delay at CT0 in SK/Nga mice but no phase shifts in C57BL/6 mice. In the present study, the possibility that the differences in the effects of pentobarbital on the circadian rhythm may be due to different contributions of the GABA-ergic system to circadian organization in the two strains was tested by comparing the responses of SK mice with those of C57BL mice to muscimol (2 mg/kg), a GABA receptor agonist, and triazolam (25 mg/kg), which is thought to act by potentiating the action of GABA. The hypothesis that pentobarbital-induced phase shifts of SK mice are mediated by the GABA receptor system was also tested by observing whether the phase-shifting effects of pentobarbital were blocked by bicuculline (0.5 mg/kg), a selective antagonist of GABA, injected 3 min prior to pentobarbital (30 mg/kg). The results indicated that muscimol induced phase advances at CT8 and phase delays at CT0, and triazolam induced phase advances at CT8 in SK mice. No phase shifts were induced by any treatment in C57BL mice. These results suggest that the role of GABA-ergic systems in circadian organization may be different in SK and C57BL mice. In addition, bicuculline could block the phase-shifting effects of pentobarbital in SK mice, suggesting that the GABA receptor system may mediate phase-shifting effects of pentobarbital in SK mice.