Abstract: The interaction of cis-[Pt(NH₃)₂Cl₂] with guanosine, adenosine, cyti-dine and thymidine, as constituents of DNA, was studied spectrophotome-trically at pH5.5,37°C in the presence of 0.1M NaClO₄. It was found that the first three nucleosides formed 1:1 and 1:2 complexes with cis-CptⅡ (NH₃)₂], whose apparent stability constants and initial rate of formation are given as follows:Thymidine showed no interaction with cis-[PtⅡ (NH₃)₂].On the basis of the above findings, a possible mode of reaction of cis-[Pt(NH₃)₂Cl₂] with DNA in the cancer cells is discussed. Thermodynami-cally and kinetically, it is considered most probable that cross links were formed between cis-[PtⅡ (NH₃)₂] and nucleosides in DNA, e. g.,guanosine,and it seems unlikely that a chelate was formed withany one nucleoside, e. g.,