Glomerular flltration represents one of the basic mechanisms in the function of an organism. Our understanding of this process is still quite fragmentary. Regulation of blood flow and pressure, together with regulation of the ultraflltration coefficient (which is an attribute of the flltration barrier), are the two fundamental mechanisms accounting for maintenance and adaptability of glomerular flltration. Regulation of glomerular blood flow is generally considered to result from an interplay between afferent and efferent glomerular arterioles, and much progress has been made recently in understanding this interplay (Navar et al. 1996). The present study provides a detailed structural description of the glomerular vascular pole of rat. The results of this study appear to be relevant for several open questions of glomerular function. First, the interaction between afferent and efferent arterioles in regulating glomerular blood is generally understood to occur between the preglomerular and the postglomerular portions of these vessels. As shown in the present study, the structural elaborations of these arterioles and the spatial relationships between them within the glomerular hilum strongly suggest an interplay also at this site. Moreover, the current understanding of glomerular blood flow regulation by tuning the interplay between afferent and efferent arterioles is exclusively based on signals whose regulatory loops are established in follow-up events outside the glomerulus (tubuloglomerular balance, tubuloglomerular feedback).