Nuclear imaging techniques can provide information about the time course of uptake, the spatial distribution, and the functional effects of a drug in the human body. Recently, PET has also acquired the potential to affect the drug development process during a very early stage, when a drug is undergoing animal testing. The development of dedicated small animal scanners with high resolution has made it possible to assess the time course of uptake of a drug within a single animal, providing that the drug is labelled with a positron-emitting isotope. Dedicated small animal scanners may therefore prove to be very useful, especially in those parts of the drug development process that require a longitudinal study design. However, in the case of receptor and enzyme studies, there may be pharmacological constraints and specific radioactivity of the radiopharmaceuticals may become an important issue. This paper will assess the potential and also the limitations of high-resolution PET in animal testing of therapeutic drugs.