Reconstructions from serial paraffin sections were used to define the relationships of the pancreatic endocrine cells and the microvasculature of the islets of Langerhans of the rat. We have reported that the afferent vessel to an islet, an arteriole, enters the islet directly in the beta-cell core. Immediately after entering the islet, this arteriole branches into capillaries. Herein, we report that beta-cells have two capillary faces. When viewed in cross section, 8-10 beta-cells form a tubelike structure around a central capillary. The outer side of each beta-cell also abuts a capillary. A clear polarity of secretory granules was seen on the ultrastructural level when the beta-cells were partially degranulated by prior in vivo treatment with glyburide. Of the remaining granules, 75.7 +/- 1.7% were accumulated in the apical portion of the cell, even though this portion was only 50.4 +/- 3.2% of the cytoplasm. beta-Cells around a particular cross-sectioned capillary shared the same orientation of granular accumulation. These findings provide morphological evidence of in situ beta-cell polarity that could be an anatomical basis for functional compartmentalization.