Seven 1,3,5- triazine (s-triazine) herbicides (ametryn, prometryn, dimethametryn, simazine, atrazine, propazine, and cyanazine) were tested using an amphibian (Silurana tropicalis) metamorphosis assay focusing on morphometric, gravimetric, and thyroid-histological endpoints. Premetamorphic tadpoles were exposed to each s-triazine at 2 concentrations between 1/1000 and 1/10 of the 96-h acute toxicity values, until all tadpoles in the control group reached either the late prometamorphosic stages or the initial stage of metamorphic climax. All s-triazines tested induced significant retardation in growth and development at the higher concentrations (0.2-1.0mg/L), and some of them induced similar effects even at the lower concentrations (0.02-0.1mg/L) while each showing a linear dose-response. Total size of the thyroid glands tended to be reduced corresponding to the delayed development, but without showing histomorphological lesions typical of anti-thyroid chemicals. These consistent results suggest that the s-triazines can act as a chemical stressor inhibiting tadpole growth and development, possibly without disrupting the thyroid axis. In addition, tadpoles exhibiting spinal curvatures appeared in either one or both of the lower and higher concentration groups for each s-triazine tested. The incidence rate in the s-triazine exposure groups where tadpoles with scoliosis were observed ranged from 3.3% to 63.3%, some of which were significantly higher than that in the respective control groups (0-6.7%). It is speculated that the s-triazines may promote to occur axial malformations in developing tadpoles.