TEMPORAL sequences of inputs to the rat whiskers are thought to be important to recognize the environment of the rat. In this study, we applied combined stimulations to neighboring whiskers D1 and D2, and the cortical activities evoked in the rat barrel cortex were measured using the intrinsic optical imaging technique. The timing of stimulation to neighboring whiskers affected the evoked cortical activities: the cortical activity evoked by in-phase stimulation to D1 and D2 was significantly stronger than that evoked by out-of-phase stimulation. In order to elucidate the mechanism underlying this phenomenon, the effect of blockade of cortical inhibitory circuits was examined. Iontophoretic application of bicuculline or saclofen (GABA-A or GABA-B antagonist) increased the evoked cortical activities and diminished the difference in activities obtained with in-phase and anti-phase stimulation. These results suggest that local inhibitory circuits play a critical role in coding temporal information of whisker stimulation. NeuroReport 10:1973-1979 (C) 1999 Lippincott Williams & Wilkins.