A prototype positron emission tomography (PET) system has been designed for high-resolution imaging of small volumes. The detectors use Pb converter stacks and multiwire proportional counters (MWPC); the data acquisition components and image reconstruction methods are also described briefly. The performance of the system is discussed in terms of sensitivity, count rate capability, spatial resolution, and scattered background. Three examples of metabolic or transport imaging demonstrate the capabilities and limitations of the system. These are blood flow to bone, cerebral glucose uptake, and nutrient translocation in plants. The performance of the prototype has been sufficiently promising that an improved system is under development.