The development of alternative methods to experimental animal models for toxicity testing is ongoing. Such alternative methods include testing on cells or tissues, also known as in vitro methods and computer-modelling techniques, also known as in silico models. A tool, which makes these alternative methods suitable for predicting toxicity is the use of physiologically based kinetic (PBK) models. Essentially, a PBK model is a “digital” body in where a simulation can be made based on mathematical equations on how a chemical is moving (absorption, distribution, metabolism, excretion = kinetics) in the body. The current thesis aimed to optimize PBK models for chemicals that are actively excreted from the body via the liver or kidneys. The optimized PBK models broaden the applicability domain of these models to predict toxicity based on in vitro/in silico toxicity testing thereby contributing to reducing, replacing and refining (3R) animal models.