The pharmacological actions of morphine and morphine-like drugs such as heroin mediate primarily through the mu opioid receptor (MOR). It represents the target of the most valuable painkiller in contemporary medicine. Here we report that poly(ADP-ribose) polymerase 1 (PARP-1) binds to the double-stranded poly(C) element essential for the MOR promoter and represses promoter activity at the transcriptional level. We identified PARP-1 by affinity column chromatography using the double-stranded poly(C) element, followed by two-dimensional gel electrophoresis and MALDI-TOF mass spectrometry. PARP-1 binding to the poly(C) sequence of the MOR gene was sequence-specific as confirmed by the supershift assay. In cotransfection studies, PARP-1 repressed the MOR promoter only when the poly(C) sequence was intact. When PARP-1 was disrupted in NS20Y cells using siRNA, transcription of the endogenous target MOR gene increased significantly. Chromatin immunoprecipitation assays showed specific binding of PARP-1 to the double-stranded poly(C) element essential for the MOR promoter. Inhibition of PARP-1's catalytic domain with 3-aminobenzamide increased endogenous MOR mRNA levels in cultured NS20Y cells, suggesting that automodification of PARP-1 regulates MOR transcription. Our data suggest that PARP-1 can function as a repressor of MOR transcription dependent on the MOR poly(C) sequence. We demonstrate for the first time a role of PARP-1 as a transcriptional repressor in MOR gene regulation.