||Glyphosate-based herbicides are widely used in agriculture and are commonly found in water bodies. Roundup Original (R) (RO) contains an isopropylamine glyphosate (GLY) salt containing the surfactant POEA, while Roundup Transorb R (R) (RTR) contains a potassium salt of GLY with unknown surfactants. Both contain different compositions of so-called “inert” ingredients, more toxic than glyphosate. Amphibian tadpoles often experience variations in O-2 availability in their aquatic habitats; an ability to tolerate hypoxia can condition their survival and fitness. We evaluated the impacts of sublethal concentrations of GLY (1 mg L-1), RO (1 mg L-1 GLY a.e) and RTR (1 mg L-1 GLY a.e) on metabolic rate ((V)over dot O-2 – mLO(2) Kg1 h(-1)) of bullfrog tadpoles during normoxia and graded hypoxia, and related this to morphology of their skin, their major site of gas exchange. In control (CT) (V)over dot O-2 remained unaltered from normoxia until 40 mmHg, indicating a critical O-2 tension between 40 and 20 mmHg. GLY significantly reduced (V)over dot O-2, possibly due to epidermal hypertrophy, which increased O-2 diffusion distance to O-2 uptake. In contrast, RTR increased (V)over dot O-2 during hypoxia, indicating an influence of “inert” compounds and surfactants. (V)over dot O-2 of RO did not differ from CT, suggesting that any increase in (V)over dot O-2 caused by exposure was antagonized by epidermal hypertrophy. Indeed, all herbicides caused marked alterations in skin morphology, with cell and epithelium wall presenting hyperplasia or hypertrophy and chromatid rupture. In summary, GLY, RO and RTR exert different effects in bullfrog tadpoles, in particular the surfactants and inert compounds appear to influence oxygen uptake. (C) 2016 Elsevier Ltd. All rights reserved.