A biomechanical model is presented for the dynamic changes in deoxyhemoglobin content during brain activation. The model incorporates the conflicting effects of dynamic changes in both blood oxygenation and blood volume. Calculations based on the model show pronounced transients in the deoxyhemoglobin content and the blood oxygenation level dependent (BOLD) signal measured with functional MRI, including initial dips and overshoots and a prolonged poststimulus undershoot of the BOLD signal. Furthermore, these transient effects can occur in the presence of tight coupling of cerebral blood flow and oxygen metabolism throughout the activation period. An initial test of the model against experimental measurements of flow and BOLD changes during a finger-tapping task showed good agreement.