The administration of the herbicide 2,4-dichlorophenyl-p-nitrophenyl ether (nitrofen; NIT) to adult female mice by i.p. injection at doses of 500 and 1000 mg/kg/day for 3 days, reduced serum thyroxine (T4) levels. T4 levels were reduced in the high-dosage group by 60% of the control and by 20% in the low-dose group. Serum triiodothyronine (T3) levels were relatively unaffected by NIT treatment at either dose. The animals appeared euthyroid, as body weights were unaffected even though T4 levels were reduced. Liver weights were slightly higher as a result of NIT exposure at both 500 and 1000 mg/kg/day.