The majority of substances listed in Table 6 have total clearances greater than those for inulin and sucrose, which implies that they are leaving the brain by routes in addition to the perivascular pathways. For those whose clearances are not much greater than 1 µL g−1 min−1, perivascular transport will still make a noticeable contribution. Na+ and Cl− enter and leave the brain parenchyma by perivascular and blood–brain barrier routes and both will be important in processes like the development and resolution of oedema (not considered in this review). Non-metabolized substances with clearances greater than about 10 µL g−1 min−1 leave the brain parenchyma primarily via transport across the blood–brain barrier.