AbstractThe effects of tributyltin(IV)chloride (TBT) on the metamorphosis of ascidian larvae of Ciona intestinalis, 2 h after hatching, were investigated. Ascidians are protochordates that lack thyroid follicles and possess thyroid hormones (THs) and their precursors, 3‐monoiodo‐tyrosine (MIT) and 3,5‐diiodo‐tyrosine (DIT), in their endostyle. According to recent findings, these hormones are also present at larval stages, localize in mesenchymal cells and their function seems to be mainly related to larval transformations. Here, we investigate the effects of TBT on thyroxine (T4) content and localization by exposing larvae of C. intestinalis for different times to TBT concentrations known to block metamorphosis. The result is a blocking of the retraction of the tail, and larval immobility. As detected by immmunohistochemistry, in normal larvae, T4 is found in all mesenchymal cells spread out in the body cavity, under the adhesive papillae and around the intestine. By contrast, in larvae exposed to 10−5 M and 10−7 M TBT, T4 is detected only in 5% and 25% of total mesenchymal cells respectively. Radioimmunoassay shows a 70% decrease of T4 content in TBT‐exposed larvae, with respect to unexposed larvae. In addition, neosynthesis of THs is inhibited. Even if parallelism does not exist between the endocrine systems of invertebrates and vertebrates, however, a close similarity exists for functions such as reproduction and metabolism. Our results indicate that TBT could behave as an endocrine disrupter (ED) in ascidians and could impair T4 metabolism. These findings suggest that the ED activity of TBT could be conserved from invertebrates to vertebrates. Copyright © 2001 John Wiley & Sons, Ltd.