Visualization and quantification of organ function by PET in small laboratory animals is becoming an outstanding tool for characterization of phenotype of transgenic and knock-out animals, for the study of animal models of human diseases, and for the development of new therapeutic drugs and diagnostic biochemical probes. To be able to make use of the PET with small laboratory animals in the same way as it is operated with humans it is necessary to account for the volumetric scale factor as well as for the requirement of maintaining the counting statistics. This work sketches the problems that these requirements represent for the technical design of the scanners and for the execution of the experiments. Finally, some characteristics of commercially available scanners (microPET/ FOCUS, HiDAC, eXplore VISTA, MOSAIC, YAP-(S)PET and rPET) are briefly discussed.