Evaluation of the Developmental and Reproductive Toxicity of Methoxychlor using an Anuran (Xenopus tropicalis) Chronic Exposure Model

The chronic toxicity of methoxychlor to the South African clawed frog, Xenopus (Silurana) tropicalis, was evaluated using a life cycle approach. The chronic exposure period ranged from mid-cell blastula stage [NF (Nieuwkoop and Faber, 1994) stage 8] to 90 days of exposure, during which time the organisms generally completedmetamorphosis and emerged as juvenile frogs.Methoxy-chlor concentrations ranged from 1 to 100 mg/l. Methoxychlor concentrations 410 mg/l caused delayed development. Organisms exposed to 10 mg/l methoxychlor for 30 days showed enlarged thyroid glands with follicular hyperplasia. No increase in mortality or external malformation was observed at any of the test concen-trations during early embryo-larval development (NF stage 8 to NF stage 46; ca. 2 days exposure). A concentration-dependent increase in external malformations and internal abnormalities of the liver and gonads were noted after 90 days of exposure, however