INCUBATION of cultured hippocampal slices with chloroquine, a compound that increases the pH of acidic subcellular organelles, for 1 0 h reduced the activity of cathepsin L by 8 3 ± 0.87% (mea n ± s.e.m.) while only marginally suppressing cathepsin B. This effect was followed within 3 h by an increase in the concentration of mature, single-chain cathepsin D (up 6 1 ± 28%). Selective depression of cathepsin L with N-CBZ L-phenylalanyl-L-phenylalanine-diazomethylketone also resulted in increases in enzymatically active cathepsin D and the delayed appearance of a 2 9 kDa fragment of the tau protein. These findings demonstrate that the pattern of cathepsin L, B, and D changes found in the aged brain can be reproduced by reducing the acidity of the lysosomal milieu. They also indicate that such pH shifts initiate a sequence of linked disturbances (inactivation of cathepsin L >induction of cathepsin D >aberrant tau proteolysis) likely to play an important role in brain ageing.