  We present a gravitational hierarchical N-body code that is designed to run
efficiently on Graphics Processing Units (GPUs). All parts of the algorithm are
executed on the GPU which eliminates the need for data transfer between the
Central Processing Unit (CPU) and the GPU. Our tests indicate that the
gravitational tree-code outperforms tuned CPU code for all parts of the
algorithm and show an overall performance improvement of more than a factor 20,
resulting in a processing rate of more than 2.8 million particles per second.
