The effects of methylmercury (CH3HgCl; MeHg) on the production of nitric oxide (NO) and the activity of the enzyme, nitric oxide synthase (NOS), in cultured human umbilical vascular endothelial cells (HUVEC) were examined. To assess the production of NO by HUVEC, platelet aggregation experiments were performed using cuvettes lined with HUVEC. Thrombin (0.05 U/ml)-induced platelet aggregation was inhibited in HUVEC pretreated with indomethacin (1 microM), an inhibitor of the cyclo-oxygenase pathway. The anti-platelet aggregatory effect of HUVEC treated with indomethacin was decreased dose-dependently by 48-h pretreatment with MeHg (0.5-2 microM). The effect of MeHg on the NADPH diaphorase staining of NOS in HUVEC showed dose-dependent cytotoxicity in regard to NOS activity. These findings suggest that MeHg inhibits the production of NO by HUVEC, an action which may be dependent on the cytotoxic effect exerted by MeHg on NOS activity.