Persistent organochlorines such as chlordecone (CD), DDT, and DDT degradation products bioaccumulate in fish and potentially impair reproduction or development via estrogenic actions. We evaluated the estrogenicity of CD, o,p'-DDT, o,p'-DDE, and p,p'-DDE injuvenile rainbow trout by assessing their potential to induce vitellogenesis; estrogen-regulated hepatic synthesis of the yolk-protein precursor, vitellogenin (Vg). In order to compare the sensitivities of various markers of estrogen stimulation, trout were injected with 17β-estradiol (0-10 mg kg-1) on days 0 and 3 and were sampled on days 3-12. Estradiol (5 mg kg-1) increased plasma Vg (2400%; 640 μg l-1), liver somatic index (200%) and hepatic cytosolic estrogen binding site levels (EBS, 300%) on day 6. These results suggested plasma Vg was the most sensitive marker of estrogen exposure. Chronic dietary exposure to CD (0.4 mg kg-1 day-1, 33 weeks) elevated plasma Vg (0.9 μg l-1), but not hepatic EBS concentrations, and resulted in relatively high hepatic CD concentrations (16 μg g-1). The in vivo estrogenicity of DDT was examined by injecting trout at 14 day intervals with single or triplicate doses of o,p'-DDT, o,p'-DDE or p,p'-DDE (0, 5, 15 or 30 mg kg-1) and monitoring vitellogenesis 14 days after the final injection. Plasma Vg and hepatic EBS concentrations were significantly elevated by o,p'-DDT and o,p'-DDE (total dose 45 and 90 mg kg-1; 23-24 μg Vg l-1) but not p,p'-DDE. Target organ doses were estimated by conducting a disposition study in which trout were injected with three doses of 14C]p,p'-DDE (30 mg kg-1), at 14 day intervals. Hepatic [14C]p,p'-DDE equivalent concentrations, 14 days after the final injection, averaged 14 μg g-1 Additionally, we evaluated the relative affinity of CD and DDT or DDE for trout hepatic EBS utilizing in vitro competitive binding assays. CD had relatively low affinity (1000-fold less than moxestrol, a synthetic estradiol) for trout hepatic EBS. o,p'-DDT and o,p'-DDE, but not p,p'-DEE also exhibited low EBS affinity (approximately 156000-fold less than moxestrol). Collectively, these results indicated that CD, o,p'-DDT and o,p'-DDE were weakly estrogenic in juvenile trout.