The term blood-brain barrier is used to describe a series of structural and functional mechanisms that control the internal environment of the central nervous system (CNS), which in the adult is distinct and remarkably stable compared with that of the rest of the body. These mechanisms operate across three main interfaces between the blood and the CNS: the cerebral blood vessels (blood-brain barrier), the choroid plexuses and the pia-arachnoid membranes (each of these latter two constitute separate blood-CSF barriers). Evidence has accumulated over the last 30 years to show that the apparent permeability of the blood-brain and blood-CSF barriers to low molecular weight (MW) lipid-insoluble compounds is greater in younger compared with older animals (see Saunders, 1992; Habgood et al. 1993; Saunders & Dziegielewska, 1997; Saunders et al. 1999). There is a widespread belief that this greater apparent permeability in the immature animal is simply a reflection of a less-well-developed barrier system (e.g. Ganong, 1999; Timbrell, 2000). Although this is probably true for some mechanisms, for others, not only are they well developed early in brain development, but the immature brain possesses additional specialised mechanisms that are not present in the adult (Fossan et al. 1985; Mollgard et al. 1987; Dziegielewska et al. 2000). Quantitative estimates of age-related changes in permeability are quite variable, even within the same species, probably because some studies were not carried out under steady-state conditions (cf. Ferguson & Woodbury, 1969; Habgood et al. 1993). Even when such conditions have been met, there have been quite different interpretations of the greater apparent permeability in younger animals. Some authors (e.g. Bass & Lundborg, 1973) have suggested that the age-related decline in blood-CSF exchange is due primarily to an increase in CSF turnover (the sink effect, see Davson & Segal, 1996), whereas others have interpreted it as a real decline in the intrinsic permeability of these interfaces (see Saunders et al. 2000 for discussion). The rate of CSF turnover in the immature brain is technically difficult to measure. Some data have been obtained for fetal sheep (Evans et al. 1974), and from these data it has been calculated that, in this species at least, the sink effect in the maturing brain actually decreases if account is taken of the total volume of CSF and the size of the brain as it grows (Saunders, 1992). We have investigated blood-CSF and blood-brain barrier permeability in very young (postnatal days (P)6–17) and older (P37–65) South American opossums (Monodelphis domestica). This is the earliest stage of brain development at which such studies have been carried out. Just how early in development this is can be illustrated by comparing the age at which the choroid plexuses appear. In fetal sheep, the choroid plexus appears in the fourth ventricle at embryonic day (E)18–21, in the lateral ventricle at E21–24, and in the third ventricle at E30–36 (Jacobsen et al. 1983). In the rat, the choroid plexuses appear at E12 (fourth ventricle), E13–14 (lateral ventricle) and E16 (third ventricle; Chamberlain, 1973). The earliest age at which permeability studies with inulin and sucrose have been carried out in these species is E40–60 in fetal sheep (Evans et al. 1974; Dziegielewska et al. 1979; Cavanagh et al. 1983) and E18 to P2 in rats (Ferguson & Woodbury, 1969; Habgood et al. 1993). In contrast, marsupials are born with only rudimentary lateral ventricular choroid plexuses, no third ventricular plexus, and only a small fourth ventricular choroid plexus (Dziegielewska et al. 2001). In addition, we have studied the rate of blood-brain and blood-CSF uptake in short-term experiments, where any age-related differences in the effect of CSF turnover on ratios are minimal. By comparing the initial rate of uptake of a lipid-insoluble (l-glucose) and a more lipid-soluble (glycerol) compound in such short-term experiments, we have been able to demonstrate a decline in barrier permeability with increasing age, a phenomenon that is independent of any change in CSF turnover (CSF sink).