Environmental risk assessment is essential but often relies on ethically controversial and expensive methods. One of the biggest current challenges in ecotoxicology is to replace animal testing with in vitro methods, for example based on cell cultures. However, evaluating whether a cell line is a suitable replacement involves testing many substances in animals and cell cultures in biologically similar conditions, i.e. the same amount of the chemical needs to be available to the cells in the culture and in animal tissues to expect the effect to be comparable. We develop physiologically based toxicokinetic (PBTK) models in fish and toxicokinetic models in cell cultures to computationally determine the chemical concentrations needed for a comparable test. Our PBTK models include physiological uptake of the chemical via breathing or food, stratification of the chemicals into vital organs and metabolization of the chemical therein. We have recently shown that, for some chemicals, exposure tests performed on the rainbow trout RTgill-W1 cell line and combined with the modeling approach can replace growth tests with juvenile fish.