Now that implantable drug infusion pumps are well established clinically, methods for diagnosing suspected pump failures are needed. The authors previously constructed a benchtop data acquisition and control apparatus to assist our work in developing new pump technology. Although this device is technically capable of in vivo pump monitoring, it is cumbersome. Thus, they recently created a portable interrogation unit with more limited features. This portable pump interrogation apparatus consists of a 32 bit MS-DOS labtop computer, data acquisition software, an analog/ASCII interface, a pressure transducer, and appropriate fluid conduits. Communication between the device and the implanted pump is via a percutaneous needle puncture of the drug reservoir refill septum. This procedure is identical to that employed in a standard pump refill. Pump performance is evaluated by incrementally filling the pump reservoir while simultaneously measuring reservoir pressure. The resulting data are presented on the computer screen as a plot of pressure versus volume that quickly and simply either eliminates or confirms the reservoir pressure source as a failure mode. Diagnostic runs are saved on file for archival purposes. Their benchtop apparatus has been a valuable and reliable tool over many years of use. The authors believe that their portable apparatus will be equally beneficial.