The complexes of triammineplatinum with uracil, 6-methyluracil, or uridine were prepared in aqueous solution at pH 7. The reaction of uracil with triammineplatinum gave two complexes at the same time. One was a complex in which triammineplatinum displaced a proton from uracil and coordinated to the N(3) position, and the other the complex coordinated to the N(1) position by displacing a proton. When triammineplatinum was treated with 6-methyluracil or uridine in aqueous solution at pH7, only a complex coordinated to the N(3) position was obtained. The ultraviolet (UV), NMR, and infrared (IR) spectral data provide useful information for determining the binding site of these complexes. The UV and IR spectral behaviors of the complex coordinated to the N(3) of uracil are very similar to those of 3-methyluracil. The NMR spectrum of the complex coordinated to the N(1) of uracil exhibits satellite peaks of 195Pt-proton and its coupling constant (39 Hz) gives good evidence for determining the binding site at the N(1) position.