The endogenous cannabinoid system is involved in the regulation of a number of physiologic effects in both the central and peripheral nervous systems. Its role in the control of neuronal cell proliferation has attracted major attention because of its potential implications for new therapeutic strategies. In the present study, we demonstrated that treatment of cultured cerebellar granule cells with the synthetic cannabinoid WIN55,212-2, causes cell-body and nuclear shrinkage, which are hallmarks of neuronal apoptosis, as well as concentration-dependent decrease in cell viability. Staining with the fluorescent nuclear dye, Hoechst 33258, revealed apoptosis in 27.1% and 58.5% of cells exposed to 1 and 10 μM of WIN55,212-2, respectively (P < 0.01 and P < 0.001 vs. control respectively) after 36 hr. After 24 hr of exposure to WIN55,212-2, mRNA levels for the anti-apoptotic gene bcl-xL were reduced to 45.6% of those found in control (P < 0.01). These effects were completely reverted when cells were exposed to the synthetic cannabinoid in the presence of the specific CB1-receptor antagonist, SR141716A (1 μM). Moreover, the pro-apoptotic effect of 10 μM WIN55,212-2 could be reduced by the addition to the incubation medium of a cell-permeant inhibitor of caspase-1 (50 nM). Finally, WIN55,212-2 significantly increased caspase-1 activity after 24 hr. These findings show that the activation of CB1 receptors on cerebellar granule cells induces apoptotic cell death, which is associated with downregulation of the anti-apoptotic gene, bcl-xL, and at least in part, activation of caspase-1.