This study investigates the decomposition of NOD by UV irradiation. Water solutions of pure NOD and NOD-containing Nodularia extract as well as Nodularia filaments collected on filters were exposed to UV-A, UV-B, and white fluorescent light (VIS) during 48 h experiments. In VIS, the toxin was fairly stable and only 3.8-4.6% of the original degraded. UV-B had the most pronounced effect on the NOD degradation rate. In the experiment, the overall loss of NOD was 0.27 and 0.77 μg ml-1 day-1 for the solution of pure toxin and Nodularia extract and 0.28 μg day-1 for Nodularia filaments. Comparison of UV-B degradation rate in water and methanol extracts revealed higher stability of NOD in methanol. This might suggest that some hydrophobic components of Nodularia cell play a protective role against UV radiation. Additionally, chemical (LC-MS/MS) and biochemical (ELISA and PPIA) assays were employed to characterize the UV degradation products. LC-MS/MS analyses showed that in UV-B exposed sample, apart from NOD, there were three other compounds with molecular ion at m/z at 825.4. The fragmentation pattern of the ion was the same for all four compounds suggesting that they are geometrical isomers of NOD. The major degradation product, with a local absorption maximum at 242 nm, was active in both biochemical assays.