Inhibitors of poly(ADP-ribose) polymerase (PARP), including benzamide, protect against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced dopamine neurotoxicity in vivo [Cosi et al., Brain Res. 729 (1996) 264–269]. In vitro, the activation of PARP by free radical damaged DNA has been shown to be correlated with rapid decreases in the cellular levels of its substrate nicotinamide adenine dinucleotide (NAD⁺), and ATP. Here, we investigated in vivo whether MPTP acutely caused region- and time-dependent changes in brain levels of NAD⁺, ATP, ADP and AMP in C57BL/6N mice killed by head-focused microwave irradiation, and whether such effects were modified by treatments with neuroprotective doses of benzamide. At 1 h after MPTP injections (4×20 mg/kg i.p.), NAD⁺ was reduced by 11–13% in the striatum and ventral midbrain, but not in the frontal cortex. The ATP/ADP ratio was reduced by 10% and 32% in the striatum and cortex, respectively, but was unchanged in the midbrain. All of these regional changes were prevented by co-treatment with benzamide (2×160 mg/kg i.p.), which by itself did not alter regional levels of NAD⁺, ATP, ADP or AMP in control mice. In a time-course study, a single dose of MPTP (30 mg/kg i.p.) resulted in maximal and transient increases in striatal levels of MPP⁺ and 3-methoxytyramine (+540%) at 0.5–2 h, followed by maximal and coincidental decreases in NAD⁺ (−10%), ATP (−11%) and dopamine content (−39%) at 3 h. Benzamide (1×640 mg/kg i.p., 30 min before MPTP) partially reduced MPP⁺ levels by 30% with little or no effect on MPTP or MPDP⁺ levels, did not affect or even slightly potentiated the increase in 3-methoxytyramine, and completely prevented the losses in striatal NAD⁺, ATP and dopamine content, without by itself causing any changes in these latter parameters in control mice. These results (1) confirm that MPTP reduces striatal ATP levels [Chan et al., J. Neurochem. 57 (1991) 348–351.]; (2) show that MPTP causes a regionally-dependent (striatal and midbrain) loss of NAD⁺; (3) indicate that the PARP inhibitor benzamide can prevent these losses without interfering with MPTP-induced striatal dopamine release; and (4) provide further evidence to suggest an involvement of PARP in MPTP-induced neurotoxicity in vivo.