Metabolism and excretion of tetrachlorvinphos were studied in dairy cows after oral administration for 5 days, followed by no treatment for a 10-day period. Orally administered tetrachlorvinphos (50 ppm, based on average daily ration intake) was absorbed, extensively metabolized, and almost totally eliminated in 10 days following the end of treatment. Traces of unchanged insecticide were detected in urine. The metabolites identified in hydrolyzed urine were as follows: desmethyl tetrachlorvinphos (13.2 % ), l-(2,4,5-trichlorophenyl)ethanol (34.8%), (2,4,5-trichlorophenyl)ethane-1,2-diol (28.1 %), and 2,4,5-trichloromandelic acid (6.1 % ). In addition, unchanged insecticide (7.1 %) as well as significant amounts of 1-(2,4,5-trichlorophenyl)ethanol (8.7%) was excreted in the feces. Neither unchanged insecticide nor metabolites could be detected in milk and various tissues from treated lactating cows.