Parkinson's disease is characterized by a loss of dopaminergic nigrostriatal neurons. This neuronal loss is mimicked by the neurotoxin 1-methyl-4-phenylpyridinium (MPP⁺). MPP⁺ toxicity is mediated through inhibition of mitochondrial complex I, decreasing ATP production, and upregulation of oxygen radicals. There is evidence that the cell death induced by MPP⁺ is apoptotic and that inhibition of caspases may be neuroprotective. In primary cultures of rat mesencephalic dopaminergic neurons, MPP⁺ treatment decreased the number of surviving dopaminergic neurons in the cultures and the ability of the neurons to take up [³H]dopamine ([³H]DA). Caspase inhibition using the broad-spectrum inhibitor benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (zVAD-fmk) spared MPP⁺-treated dopaminergic neurons and increased somatic size. There was a partial restoration of neurite length in zVAD-fmk-treated cultures, but little restoration of [³H]DA uptake. Peptide inhibitors of caspases 2, 3, and 9, but not of caspase 1, caused significant neuroprotection. Two novel caspase inhibitors were tested for neuroprotection, a broad spectrum inhibitor and a selective caspase 3 inhibitor; both inhibitors increased survival to >90% of control. No neuroprotection was observed with an inactive control compound. MPP⁺treatment caused chromatin condensation in dopaminergic neurons and increased expression of activated caspase 3. Inhibition of caspases with either zVAD-fmk or a selective caspase 3 inhibitor decreased the number of apoptotic profiles, but not expression of the active caspase. We conclude that MPP⁺ toxicity in primary dopaminergic neurons involves activation of a pathway terminating in caspase 3 activation, but that other mechanisms may underlie the neurite loss.