In general, gradient-echo (GE) BOLD contains extravascular (EV) contributions from all sized vessels, while spin-echo (SE) BOLD is sensitive to microvessels. Based on simulation, the EV BOLD signal is dependent linearly on B0 for macrovessels, and quadratically on B0 for microvessels. Here, we performed GE and SE BOLD fMRI of α-chloralose anesthetized rats responding to forepaw stimulation on an ultrahigh magnetic field of 15.2T. Stimulation-induced R2 change was quadratically on B0, indicating that microvessel contributions are dominant. SE BOLD at ultrahigh fields can detect precise activation sites and can be used for high-resolution fMRI to detect fine functional structures.