The perchlorate anion inhibits thyroid hormone (TH) synthesis via inhibition of the sodium-iodide symporter. It is, therefore, a good model chemical to aid in the development of a bioassay to screen chemicals for affects on thyroid function. Xenopus laevis larvae were exposed to sodium perchlorate during metamorphosis, a period of TH-dependent development, in two experiments. In the first experiment, stage 51 and 54 larvae were exposed for 14 d to 16, 63, 250, 1,000, and 4,000 microg perchlorate/ L. In the second experiment, stage 51 larvae were exposed throughout metamorphosis to 8, 16, 32, 63, and 125 microg perchlorate/L. Metamorphic development and thyroid histology were the primary endpoints examined. Metamorphosis was retarded significantly in the first study at concentrations of 250 microg/L and higher, but histological effects were observed at 16 microg/L. In the second study, metamorphosis was delayed by 125 microg/L and thyroid size was increased significantly at 63 microg/L. These studies demonstrate that inhibition of metamorphosis readily can be detected using an abbreviated protocol. However, thyroid gland effects occur at concentrations below those required to elicit developmental delay, demonstrating the sensitivity of this endpoint and suggesting that thyroidal compensation is sufficient to promote normal development until perchlorate reaches critical concentrations.