The design of the microvasculature of cerebellum and nontegmental rhombencephalic areas was studied in eight adult Acipenser ruthenus L. by scanning electron microscopy of vascular corrosion casts and three-dimensional morphometry. Gross vascularization was described and diameters and total branching angles of parent and daughter vessels of randomly selected arterial and capillary bifurcations (respectively, venous mergings) were measured. With diameters ranging from 15.9 ± 1.9 μm (cerebellum; mean ± S.D.) to 15.9 ± 1.7 mm (nontegmental rhombencephalon; mean ± S.D.) capillaries in Acipenser were significantly (p ≥ .05) smaller than in cyclostomes (18–20 μm) but significantly thicker than in higher vertebrates and men (6–8 μm). With the exception of the area ratio β (i.e., sum of squared daugther diameters divided by squared diameter of parent vessel) of the venular mergings in the nontegmental rhombencephalon, no significant differences (p ≥ .05) existed between the two brain areas. Data showed that arteriolar and capillary bifurcations and venular mergings are optimally designed in respect to diameters of parent vessel to daughter vessels and to branching (merging) angles. Quantitative data are discussed both in respect to methodical pitfalls and the optimality principles possibly underlying the design of vascular bifurcations/mergings in selected brain areas of a nonteleost primitive actinopterygian fish.