Purpose: The aim of this study was to examine the effects of attenuation correction (AC) and scatter correction (SC) on the quantification of PET count rates. Materials and Methods: To assess the effects of AC and SC, 18F-FDG PET images of phantom and cat brain were acquired using microPET R4 scanner. Thirty-minute transmission images using 68Ge source and emission images after injection of FDG were acquired. PET images were reconstructed using 2D OSEM. AC and SC were applied. Regional count rates were measured using ROIs drawn on cerebral cortex including frontal, parietal, and latral temporal lobes and deep gray matter including head of caudate nucleus, putamen and thalamus for pre- and post-AC and SC images. The count rates were then normalized with the injected dose per body weight. To assess the effects of AC, count ratio of "deep gray matter/cerebral cortex" was calculated. To assess the effects of SC, ROIs were also drawn on the gray matter (GM) and white matter (WM), and contrast between them ((GM-WM)/GM) was measured. Results: After the AC, count ratio of "deep gray matter/cerebral cortex" was increased by 17¡¾7%. After the SC, contrast was also increased by 12¡¾3%. Conclusion: Relative count of deep gray matter and contrast between gray and white matters were increased after AC and SC, suggesting that the AC would be critical for the quantitative analysis of cat brain PET data. (Nucl Med Mol Imaging 2006;40(1):40-47)