Phase delays of hamster locomotor rhythms, which normally can be produced by light pulses given in the early subjective night, were blocked by bicuculline, a selective antagonist of γ-aminobutyric acid (GABA) activity¹⁷. Phase advances induced by light pulses given in the late subjective night were unaffected by bicuculline. This suggests that GABA may be involved in the mediation of some, but not all, light input to the mammalian circadian system. Furthermore, it raises the possibility that light input may be mediated at different circadian time points by functionally separate pathways.