We have investigated the dynamic response of rodent posteromedial barrel subfield (PMBSF) cortex to mechanical whisker deflection, using optical intrinsic signal imaging. While electrophysiologic response in barrel cortex has been well studied, dynamic metabolic changes affecting activity-related perfusion or oxidative enzymes are not well understood. Male Sprague-Dawley rats were anesthetized. Contralateral single and multiple vibrissae were deflected while images of somatosensory cortex were acquired with a charge-coupled-device camera. Intrinsic signals were observed over PMBSF as stimulus-related reflectance decreases (10(-3) of baseline) comprising two distinct spatiotemporal components. At 610 nm the first, diffuse, component begins 0.5-1 sec after stimulus onset, peaks at 2.5-3 sec, and returns to baseline by 4-5 sec. The second component is macrovascular, beginning at 1-1.5 sec, peaking at 3 sec, and dissipating by 5-6 sec. Similar patterns were observed at 550 nm and 850 nm. Signal size and location varied with the stimulus. Evoked potentials were found to have maximal amplitude in the region of maximal optical signals, diminishing toward the optical periphery. We have demonstrated PMBSF response to vibrissal deflection using optical reflectance methods. These intrinsic signals overlie regions of maximal electrophysiologic response, but commence, peak, and extinguish over a time scale of seconds from stimulus onset. They most likely indicate activity-related microvascular recruitment and chromophore redox changes.