Functional mapping of the rat somatosensory cortex was performed with T2*-sensitized MRI using a forepaw electrical stimulation model in alpha-chloralose-anesthetized rats at 7 T under both normocapnia and mild hyperventilation-induced hypocapnia. A highly localized activation area, consistent with the known somatosensory cortical region, was detected in all seven animals studied during hypocapnia and in five of the same animals during normocapnia. Quantitatively, hypocapnia was found to significantly increase both the size of the fMRI activation area (3.4 +/- 0.6 mm2 versus 1.5 +/- 0.6 mm2 in normocapnia, mean +/- standard error, n = 7, P < 0.03) and the average fMRI signal intensity increase (3.4 +/- 0.6% versus 2.7 +/- 0.4%, n = 5, P < 0.05). The increased sensitivity of fMRI to functional activation may reflect a widened arterial-venous oxygenation difference resulting from an increased effective oxygen extraction during hyperventilation. The dependence of the fMRI response on the ventilation state underscores the need to control for physiological parameters in animal fMRI studies.