The primary lesion in Alzheimer’s disease and dialysis dementia has been postulated to be an impaired blood-brain-barrier (BBB) permeability that allows neurotoxins like aluminium to reach the central nervous system. The present study shows that aluminium itself affects the permeability of the BBB of rats to small peptides. Intraperitoneal injection of aluminium chloride increased the permeability of the BBB to iodinated N-Tyr-delta-sleep-inducing peptide and beta-endorphin by 60-70%. Thus, aluminium can affect the BBB in ways that might be involved in dementia.