Evaluation of population-level impacts is critical to credible ecological risk assessments. In this study, a predictive model was developed to translate changes in fecundity of the fathead minnow (Pimephales promelas) in a short-term laboratory toxicity test to alterations in population growth rate. The model uniquely combines a Leslie population projection matrix and the logistic equation. Application of the model requires only a life table for the organism of interest, a measure of carrying capacity for the given population, and an estimation of the effect of a stressor on vital rates. The model was applied to investigate population dynamics for fathead minnow exposed to the androgen receptor agonist 17beta-trenbolone. Organismal-level responses for fathead minnows exposed to varying levels of 17beta-trenbolone were used to determine projected alterations in a population existing in a small body of water containing varying concentrations of the androgen. Fathead minnow populations occurring at carrying capacity and subsequently exposed to 0.027 microg/L of 17beta-trenbolone exhibited a 51% projected decrease in average population size after 2 years of exposure. Populations at carrying capacity exposed to concentrations of 17beta-trenbolone > or = 0.266 microg/L exhibited a 93% projected decrease in average population size after 2 years of exposure. Overall, fathead minnow populations exposed to continued concentrations of 17beta-trenbolone equal to or greater than 0.027 microg/L were projected to have average equilibrium population sizes that approached zero.