A high amplitude slow wave component of frequency 0.005 to 0.012 Hz was observed in multiple resting state fMRI scans of the rat brain under different anesthetics. These ultra-slow waves were localized predominantly in the hypothalamus. We hypothesize that these ultra-slow oscillations might have a neurological origin and may contribute to the analysis of resting state connectivity, which is currently limited to frequencies above 0.01 Hz . Future studies should investigate the origin of these oscillations with an independent measure (e.g. calcium imaging) and the implications of their incorporation into the analysis of rs-fMRI.