Brief cortical neural activity creates changes in local blood flow and oxygen uptake. Functional magnetic resonance imaging can measure this neurovascular coupling as a blood oxygen level dependent (BOLD) signal. The BOLD response to brief stimulation is termed the hemodynamic response function (HRF). We developed a computational model for the BOLD HRF, which predicts that the flow component of the HRF is a simple underdamped sinusoid. To test this prediction, we used arterial spin labeling to measure both CBF and BOLD responses in human cortex with high spatial and temporal resolution. Results confirm a significant flow undershoot in five subjects.