Sodium nitroprusside (SNP), a nitric oxide donor, or centrifugation at 150 times unit gravity, caused a nitric oxide burst in oocyte-derived Taxus brevifolia haploid cultures. This burst, visualized by the specific fluorescent probe 4,5-diaminofluorescein diacetate (DAF-2 DA), preceded a significant increase in nuclear DNA fragmentation and cell death. DNA fragmentation was detected in situ by the terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) of DNA 3'-OH groups. Nitric oxide formation and cell death were significantly decreased by N(G)-monomethyl-L-arginine (L-NMMA), a nitric oxide-synthase (NOS; EC 22.214.171.124) inhibitor. Our results show that nitric oxide leads to irreversible DNA fragmentation and cell death under stressful conditions, and that its effect can be prevented by L-NMMA.