During the last decade, image-processing techniques have been introduced as a valuable tool in biologically oriented research. In combination with novel fluorescent probes, these techniques permit assessment of subcellular distributions of several intracellularly important cations, such as free calcium ions and protons. Typically, systems used for image processing are located centrally around the experimental setup. This configuration has drawbacks, mainly because the laborious extraction and processing of data that generally follow an experimental session limits the access to the system for other investigators. We describe here the principles of a distributed image processing system, based on IBM-compatible personal computers (PCs), that without extra hardware can cope with all the necessary image processing involved in imaging of intracellular cations. The potential of the PC as an image processor, however, reaches beyond this specific application and many image processing tasks can be carried out successfully on a standard PC. Thus, the centrally located dedicated image processor is used only for image acquisition in the experimental situation. This in turn optimizes the utilization of expensive resources and increases efficiency. The mouse-operated software is described in detail, so that interested investigators can extract useful parts for integration into their own applications and experimental environment.