The study investigates the amplitude of spontaneous low-frequency oscillations (LFO) in the human resting brain using fMRI and examines the test-retest reliability of these amplitude measures. Key findings include: 1. **Amplitude Distribution**: Gray matter exhibits higher LFO amplitudes than white matter, with the largest amplitudes observed in mid-brain structures associated with the "default-mode" network. High amplitude LFO activity in specific brain regions is reliable across time. 2. **Reliability**: Both Amplitude of Low Frequency Fluctuations (ALFF) and fractional Amplitude of Low Frequency Fluctuations (fALFF) exhibit moderate to high intra- and inter-session test-retest reliability, particularly in gray matter. 3. **Regional Differences**: Significant and highly reliable ranking orders of LFO amplitudes among anatomical parcellation units are observed. Visual cortex, posterior cingulate cortex, thalamus, medial prefrontal cortex, and anterior cingulate cortex show the highest amplitudes. 4. **Frequency Band Analysis**: Detailed examination of individual low-frequency bands reveals distinct spatial profiles. Slow-4 (0.027 - 0.073 Hz) band amplitudes are most robust in the basal ganglia, consistent with spontaneous electrophysiological recordings in awake rats. 5. **Clinical Implications**: LFO amplitude measures may contribute to further characterization of existing and future "resting-state" fMRI datasets, providing a potentially meaningful and stable property of the human brain. The study suggests that LFO amplitude measures can be valuable for understanding brain dynamics and may have clinical applications in neuroimaging studies.The study investigates the amplitude of spontaneous low-frequency oscillations (LFO) in the human resting brain using fMRI and examines the test-retest reliability of these amplitude measures. Key findings include: 1. **Amplitude Distribution**: Gray matter exhibits higher LFO amplitudes than white matter, with the largest amplitudes observed in mid-brain structures associated with the "default-mode" network. High amplitude LFO activity in specific brain regions is reliable across time. 2. **Reliability**: Both Amplitude of Low Frequency Fluctuations (ALFF) and fractional Amplitude of Low Frequency Fluctuations (fALFF) exhibit moderate to high intra- and inter-session test-retest reliability, particularly in gray matter. 3. **Regional Differences**: Significant and highly reliable ranking orders of LFO amplitudes among anatomical parcellation units are observed. Visual cortex, posterior cingulate cortex, thalamus, medial prefrontal cortex, and anterior cingulate cortex show the highest amplitudes. 4. **Frequency Band Analysis**: Detailed examination of individual low-frequency bands reveals distinct spatial profiles. Slow-4 (0.027 - 0.073 Hz) band amplitudes are most robust in the basal ganglia, consistent with spontaneous electrophysiological recordings in awake rats. 5. **Clinical Implications**: LFO amplitude measures may contribute to further characterization of existing and future "resting-state" fMRI datasets, providing a potentially meaningful and stable property of the human brain. The study suggests that LFO amplitude measures can be valuable for understanding brain dynamics and may have clinical applications in neuroimaging studies.