Cadmium is a ubiquitous pollutant in aquatic environments that can alter organismal physiology and ecology. Previous experiments found that ecological Cd exposures increased the growth and development of two North American anurans. However, the generality of these effects among species, the time period over which they occur, and the mechanisms responsible remain conjectural. The goal of the present study was to determine the critical period of sensitivity of Rana pipiens exposed to ecologically relevant levels of Cd. We exposed tadpoles to Cd (0 [control], 1.0, and 10.0 microg/L) from Gosner stage (GS) 25 to metamorphic climax. We assessed effects of Cd on amphibian length, survival, and development during premetamorphosis (GS 25-30) and prometamorphosis (GS 31-42). After 14 d of exposure, we staged tadpoles and recorded snout-vent length. Tadpoles were then pooled according to treatment and stage (GS < or =29 or GS > or =30) and allowed to undergo metamorphic development. Tadpoles exposed to 10 microg/L were significantly larger and more advanced in development by 14 d. Survival to forelimb emergence exceeded 90% in all treatments, and time to metamorphic climax was not different from that in controls. Body burdens of Cd were positively correlated with increasing treatment. Early amphibian development (premetamorphosis) was shown to be the critical period of sensitivity for growth and development. Whereas the freshwater criterion for Cd appears to be protective for survival, a lack of knowledge remains about the sublethal effects of chronic exposures of metal pollutants, especially as they relate to tissue concentrations at various stages of amphibian life history.