This study of the Black bear (Ursus amerícanus) was undertaken to provide basic information related to structural modifications in the renal microvasculature that might provide insight into the drastic alteration in renal urinary output that occurs during winter sleep. Vascular casts, as well as light microscopy and scanning electron microscopy, were used to study the vascular components of the juxtaglomerular complex and related vessels. Histologically, arterial cushions were readily identified at the origin of the afferent arterioles. In the area of the juxtaglomerular complex, the wall of the afferent arteriole appeared to be highly modified. The smooth muscle cells at this site demonstrated a change in morphology and orientation, and the diameter of the arteriole was altered. The pattern of the vascular casts at the origin of the afferent arteriole varied from that portion at the glomerulus, suggesting a modification of the vascular wall near the renal corpuscle. Although the morphology of the renal microvasculature of the Black bear is similar to that of other mammals in some aspects, it is dissimilar to that of other carnivores and of the human kidney in that there are structural modifications of the afferent arteriole that may contribute to a reduction of blood flow to the nephron during winter sleep