Despite the ubiquitous use of functional magnetic resonance imaging (fMRI), the extent to which the magnitude and spatial scale of the fMRI signal correlates with neuronal activity is poorly understood. In this study, we directly compared single and multiunit neuronal activity with blood oxygenation level-dependent (BOLD) fMRI responses across a large area of the cat area 18. Our data suggest that at the scale of several millimeters, the BOLD contrast correlates linearly with the underlying neuronal activity. At the level of individual electrode recording sites, however, the correlation between the two signals varied substantially. We conclude from our study that T(2)*-based positive BOLD signals are a robust predictor for neuronal activity only at supra-millimeter spatial scales.