Metabolism of xenobiotics is often seen as an exclusive function of the liver, but some current findings support the notion that the lungs, kidneys and intestine may contribute considerably. After the establishment of the use of liver slices as a useful in vitro model to study metabolism and toxicity of xenobiotics, the same concept is currently being used for slices from lung, kidney and intestine. It is the aim of this review to discuss the use of organ slices in biotransformation research. The basic idea behind the use of tissue slices in biomedical research is the assumption that the cells under study will function optimally in vitro if they are cultivated in an environment that is most alike to their natural in vivo embedding, which is the case in tissue slices. Advantages in the use of organ slices are the relatively easy preparation as well as the potential standardization of both the preparation and use. Moreover, a direct interspecies comparison can be made between liver, lungs, kidneys and intestines, for example with respect to their metabolic capacity and their sensitivity for toxicants. Of major importance is that organ slices can be made with a similar procedure from organs/tissues originating from different species, including man. This latter aspect is useful in drug development in general but also for a better insight in the metabolic fate of compounds in man. Importantly the use of slices may largely contribute to a reduction in the use of experimental animals.