This report describes the identification of diquat-dipyridone and diquat-monopyridone as metabolites of diquat, and time course changes of these metabolites plus diquat and paraquat in urine and serum of three poisoned patients who ingested the combination herbicide Preeglox L (containing 5% paraquat dichloride and 7% diquat dibromide) in a suicide attempt. Diquat-dipyridone was isolated from urine and identified by electron impact mass spectrometry, thin layer chromatography and high performance liquid chromatography (HPLC). Diquat-monopyridone, diquat-dipyridone, diquat and paraquat in serum and urine were assayed by HPLC with fluorescence and UV detection. Paraquat-monopyridone as a metabolite of paraquat was not detected in any samples. These results indicate that diquat is metabolized to diquat-monopyridone and diquat-dipyridone and also that the metabolism of diquat is easier than that of paraquat in humans. The concentrations of each metabolite were always lower than that of diquat in each specimen. However, the ratios of the concentration of each metabolite against the concentration of diquat increased with the decrease of diquat concentration. The metabolism of diquat seems to lower slightly the diquat concentration.