The persistence of terbuthylazine, simazine, atrazine and prometryn (s-triazine herbicides) was studied in sea, river and groundwaters during long-term laboratory incubation (127 days) under different laboratory conditions (light-darkness at 20 degrees C). Analysis of herbicides was performed by GC-NPD and their identity was confirmed by GC-MSD. A micro on-line method for the isolation of herbicide residues was used. The results showed that light had little effect on the removal of the four herbicides in riverwater but had a marked effect on their removal from sea and groundwater. Surprisingly, this removal appeared to be inversely proportional to the concentration of dissolved organic materials. In general, the degradation order was similar in sea and riverwaters; simazine was the most readily degraded compound (t(1/2)= 29-49 days), while terbuthylazine was the most persistent with the longest half-lives (76-331 days). In groundwater, terbuthylazine also showed greater persistence but prometryn was the compound with a fastest degradation rate, half-lives ranged from 88 days for prometryn to approximately 100 days for the other three compounds in light conditions and 263-366 days for prometryn and terbuthylazine, respectively, in darkness. Only for terbuthylazine was the remaining percentage at the end of the experiment higher than 50% under light conditions in riverwater, while in the other cases, the remaining percentage varied from 7 to 43% for simazine in seawater and atrazine in groundwater, respectively. Finally, a greater persistence was observed in groundwater for the four compounds.