Exposure of mouse thymocytes to dopamine caused apoptosis (programmed cell death). This was manifested by cellular condensation and membrane damage shown by flow cytometry measurements and scanning electron microscopic study. Dopamine also affected thymocytic nuclei and their genomic DNA integrity. Most of the DNA molecules accumulated in a subdiploid peak in flow cytometry analysis, indicating DNA fragmentation to small particles. DNA analysis showed the typical pattern of 'DNA ladder' caused by internucleosomal DNA cleavage. X-ray microanalysis of the cellular elements of dopamine-treated cells showed elevation of sodium (Na), chloride (Cl) and calcium (Ca) peaks, accompanied by reduction in phosphate (P) concentrations. Comparison of the potassium (K) and P concentrations showed significant differences between the two major death processes: necrosis (induced by exposure to sodium azide (NaN3)) and apoptosis (induced by dopamine). High concentrations of K indicated cell viability while reductions in P and elevations in Ca levels were found to be typical of apoptotic cell death. The antioxidant dithiothreitol (DTT) suppressed dopamine-induced apoptosis in thymocytes, suggesting that its toxicity may be mediated via generation of reactive oxygen radicals. Our study suggests that under certain circumstances, dopamine and/or its metabolites, may induce a process of apoptotic cell death of the dopamine-producing cells in the substantia nigra. Increased accessibility of dopamine to the nigral cell nucleus or inability to scavenge excess free radicals generated from dopamine oxidation triggering programmed cell death, may cause the progressive nigral degeneration in Parkinson's disease.