This study investigated the effects of 2,3,7,8-tetrachlorodibenzo-rho-dioxin (TCDD) and estrogen on plasma lipids in immature male chickens. Fatty acids were quantified in plasma collected on day 14 from chickens injected with either: Estrogen plus TCDD-1 mg estradiol cypionate /kg body wt. daily for 3 days and 50 microg TCDD/kg body wt. on day 4; Estrogen--1 mg estradiol cypionate/kg body wt. daily for 3 days and vehicle only on day 4; TCDD-vehicle only for 3 days and 50 microg TCDD/kg body wt. on day 4; or Vehicle--same volume of appropriate vehicle for 4 days. TCDD treatment alone increased the plasma concentrations of total triacylglycerides and of the specific fatty acids 14:0, 15:0, 18:0, 18:2n6, 18:3n3, 20:0, 20:1n9, 20:2n6, 20:3n6, 20:5n3 and 22:1n9, compared with vehicle treatment. The concentration of 22:6n3 was increased in all plasma lipid classes of the estrogen group compared with the vehicle group, but was not increased in the estrogen plus TCDD group. Overall, TCDD treatment alone increased plasma lipids, possibly as a result of decreased clearance or utilization; whereas estrogen plus TCDD treatment antagonized estrogen-induced increases in 22:6n3 but did not cause hyperlipidemia.