In pentobarbital-anesthetized male Sprague-Dawley rats, a small cranial window was trephined, and the cortex was transilluminated with a fine glass fiber inserted into the brain parenchyma. The light intensity at the surface area of 2 x 2 mm was recorded during intracarotid injection of 25 microl of carbon black (CB) solution. The region of interest (ROI) was divided into a 50 x 50 matrix, and the mean transit time of CB transport was calculated in each matrix element. We found rapid transits of CB along the microvasculature, with considerable heterogeneity in the avascular area, and heterogeneous efficiency in autoregulatory capacity in the ROI during hypotension. The method was validated by comparison with laser-Doppler flowmetry. The average mean difference was 0.03 +/- 0. 05%. Five percent CO(2) inhalation increased the flow by 85%, but heterogeneously. We concluded that the technique is exclusively sensitive to indicator transits in a very small area on the brain surface with potential usefulness in detecting regional heterogeneity in blood flow.