Female mink were exposed to a technical polychlorinated biphenyl (PCB) preparation (Clophen A50 [A50]; 0.1 or 0.3 mg/animal/d), one fraction of A50 containing the non- and mono-ortho-chlorinated congeners (0–1-ortho-chlorobiphenyls [CBs]), another fraction of A50 containing the congeners with two to four ortho-chlorines (2–4-ortho-CBs), or an organic extract from Baltic gray seal blubber. The animals were exposed for 18 months, including two reproduction seasons. Among the animals given the highest dose of A50, the whelping frequency was reduced in the second reproductive season, and all kits died within 24 h of birth. Reproduction was also impaired by the lower dose of A50. Daily exposure to the 0–1-ortho-CBs separated from 0.3 mg A50 severely reduced kit survival. Reproduction was not significantly impaired by daily exposure to the 2–4-ortho-CBs separated from 0.3 mg A50 or by exposure to the blubber extract. We conclude that the reproductive toxicity in chronically PCB-exposed mink is caused by the aryl hydrocarbon (Ah) receptor agonists. The lowest-observed-effect level for reproductive impairment was 2.4 ng 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) equivalents (TEQs) per kilogram body weight and day (22 pg TEQs/g feed). Ethoxyresorufin-O-dealkylase (EROD) was strongly induced by the 0–1-ortho-CBs and pentoxyresorufin-O-dealkylase by the 2–4-ortho-CBs. High EROD activity was correlated with low kit production, and consequently EROD may serve as a marker for reproductive toxicity by Ah receptor agonists in mink.