The permeability of the blood-brain barrier (BBB) to methotrexate, Na+, and Cl- was studied in rats treated with the membrane-active antifungal agent amphotericin B. Enhancement of brain uptake of methotrexate was expected as amphotericin B increases the permeability of lipid membranes, including tumor cell membranes. However, amphotericin B had no effect of the BBB permeability to methotrexate, Na+, and Cl-. This may have been because the amphotericin B acted only on the luminal and not on the abluminal brain capillary membrane, leaving the BBB intact.