Purpose: The capacity limitation of human attention is best exemplified in attentive tracking of moving objects: our tracking ability declines when more objects are tracked, or when each object moves at a faster speed. Previous behavioral studies showed a trade-off between the number of objects tracked and the speed of each object, suggesting that a single type of attention is loaded. Using fMRI we examined whether common or separate neural substrates underlie spatial and non-spatial load. Methods: We parametrically manipulated two types of attentional load using an attentive tracking task. A rotating pinwheel was presented at each of four visual quadrants simultaneously. Observers tracked the cued spoke of a pinwheel(s) with attention while maintaining central fixation. We manipulated spatial load by cuing observers to monitor 1 or 2 pinwheels, and non-spatial load by varying the rotation speed of each wheel. Results: Tracking accuracy deteriorated as each wheel rotated more rapidly, with a sharper decline when more wheels were tracked. Posterior parietal cortex, however, was sensitive only to spatial load. Its activation was higher when subjects tracked 2 rather than 1 wheel, but showed no systematic change at different rotation speeds (from 100-400deg/s). In addition, although tracking accuracy was higher in bilateral tracking than unilateral tracking of two wheels, posterior parietal activity was equivalent between these two conditions. Conclusion: These results suggest that spatial and non-spatial attentional load are represented differently in the brain, with the posterior parietal cortex more sensitive to spatial attentional load than to non-spatial attentional load.