Kinesin is a microtubule-based motor protein responsible for anterograde transport of vesicles and organelles in nerve axons and other cell types. The energy necessary for this transport is derived from the hydrolysis of ATP which is thought to induce conformational changes in the protein. We have solved the X-ray crystal structures of rat brain kinesin in three conditions intended to mimic different nucleotide states: (1) with ADP bound to the nucleotide-binding site, (2) with bound ADP in the presence of AIF4-, and (3) with ADP hydrolyzed to AMP by apyrase. In contrast to analogous cases observed in GTP-binding proteins or the muscle motor myosin, the structure of kinesin remained nearly unchanged. This highlights the stability of kinesin's ADP state in the absence of microtubules. Surprisingly, even after hydrolysis of ADP to AMP by apyrase a strong density peak remains at the position of the β-phosphate which is compatible either with a phosphate or a sulfate from the solvent and appears to stabilize the nucleotide-binding pocket through several hydrogen bonds.