We used an intrinsic optical imaging technique to examine postnatal developmental changes in the rat barrel response to a single whisker movement. We compared the optical response patterns between control and de-whiskered rats, from which whiskers were removed except for the D1 whisker just after birth. Barrel responses were evoked by D1-whisker movement stimulation, and the intrinsic optical signals were detected from the somatosensory cortex through the dura mater. In the control rats, the area of the barrel response decreased gradually as postnatal development proceeded from 2 to 7 wk, until reaching the adult pattern. On the other hand, in the de-whiskered rats, the barrel response area did not change during development and showed a larger size than in the control rats. We also compared the trial-to-trial variations in the barrel responses between the two groups. In the control rats, trial-to-trial variations in the optical responses were observed under the same conditions of whisker stimulation, and the extent of the variations decreased with postnatal development up to 7 wk. In the de-whiskered rats, trial-to-trial variations were also observed, but the extent was larger and unchanged during development. In both groups, the positions of the response area were the same with respect to the bregma. These results suggest that the decrease in the area and variations in the optical responses are caused by interactions of the corresponding whisker barrel with neighboring barrels and that these interactions are necessary for the developmental stabilization of the intracortical horizontal connections, which are widespread and have high plasticity in early postnatal periods.