On 50 human brains the width and course of the veins on the surface of the mesencephalon were determined. The basal vein, for example, has an inside diameter of 1.70 (0.91-2.88) mm. A lateral mesencephalic vein could be demonstrated in 70% of the cases, the width being 0.81 (0.29-1.78) mm. A posterior mesencephalic vein was found in 51%, measuring 0.57 (0.24-1.01) mm in diameter. The ponto-mesencephalic vein and the intercollicular vein were also included in this examination.