The psychoactive ingredient of marijuana, Delta9-tetrahydrocannabinol (Delta9-THC), can evoke apoptosis in cultured cortical neurones. Whilst the intracellular mechanisms responsible for this apoptotic pathway remain to be fully elucidated, we have recently identified a role for the CB1 type of cannabinoid (CB) receptor and the tumour suppressor protein, p53. In the current study, we demonstrate the Delta9-THC promotes a significant increase in lysosomal permeability in a dose- and time-dependent manner. The increase in lysosomal permeability was blocked by the CB1 receptor antagonist, AM251. Delta9-THC increased the localization of phospho-p53Ser15 at the lysosome and stimulated the release of the lysosomal cathepsin enzyme, cathepsin-D, into the cytosol. The p53 inhibitor, pifithrin-alpha and small interfering RNA-mediated knockdown of p53 prevented the Delta9-THC-mediated increase in lysosomal permeability. Furthermore, the Delta9-THC -mediated induction of apoptosis was abrogated by a cell-permeable cathepsin-D inhibitor (10 microM). Thus, the study demonstrates that Delta9-THC impacts on the lysosomal system, via p53, to evoke lysosomal instability as an early event in the apoptotic cascade. This provides evidence for a novel link between the CB1 receptor and the lysosomal branch of the apoptotic pathway which is crucial in regulating neuronal viability following exposure to Delta9-THC.