Plasmonic waveguides offer promise in providing a solution to the bandwidth limitations of classical electrical interconnections(1-3). Fast, low-loss and error-free signal transmission has been achieved in long-range surface plasmon polariton waveguides(4,5). Deep subwavelength plasmonic waveguides with short propagation lengths have also been demonstrated(6,7), showing the possibility of matching the sizes of optics and today's electronic components. However, in order to combine surface plasmon waveguides with electronic circuits, new high-bandwidth electro-optical transducers need to be developed. Here, we experimentally demonstrate the electrical detection of surface plasmon polaritons in metallic slot waveguides. By means of an integrated metal-semiconductor-metal photodetector, highly confined surface plasmon polaritons in a metal-insulator-metal waveguide are detected and characterized. This approach of integrating electro-optical components in metallic waveguides could lead to the development of advanced active plasmonic devices and high-bandwidth on-chip plasmonic circuits.