In this study we investigated the acute exposure of the aquatic macrophyte Callitriche obtusangula to the herbicide oxadiazon (Ronstar). The toxic effects on C. obtusangula were evaluated, 24h after exposure, by assessing visible necrotic leaf lesions and, 12 h after exposure, via analyses of dead cells and hydrogen peroxide (H2O2) deposits localized by histocytochemical analysis with Trypan blue and 3,3'-diaminobenzidine (DAB), respectively. As a result, we found that 0.1275 microg L(-1) a.i. (active ingredient) oxadiazon was the maximum concentration that produced no observable adverse effects (NOAEC) both at leaf and tissue levels, at any considered exposure time. Additionally, we assayed the protective effect of pre-treatment with 0.25 mM N-acetylcysteine (NAC), a cysteine donor, on the damage caused by the toxic herbicidal dose of 6.37 microg L(-1) a.i to C. obtusangula, correlating the NAC observed protection to the direct H2O2-scavenging and to the enhancement of glutathione parameters. NAC-treated plants showed a fourfold increase in the GSH (reduced glutathione)+GSSG (oxidised glutathione) content (149.2 nmol g(-1) FW) compared to controls (36.1 nmol g(-1) FW); in the NAC+oxadiazon treatments, the GSH+GSSG content was more than fivefold higher (202.1 nmol g(-1) FW). GSH showed a similar trend in NAC and NAC+oxadiazon treatments, being six- (130.0 nmol g(-1) FW) and eightfold (185.0 nmol g(-1) FW) higher, respectively, compared to controls (20.7 nmol g(-1) FW). Accordingly, the GSH/GSSG ratio in NAC- and NAC+oxadiazon-treated plants was significantly increased compared to controls, indicating alleviation of oxidative stress.