All living organisms are made of cells and they store their hereditary information in the form of double stranded DNA. In all organisms DNA replication and repair is essential for cell division and cell survival. These processes require deoxyribonucleotides (dNTPs), the building blocks of DNA. Ribonucleotide reductase (RNR) is catalyzing the rate limiting step in the de novo synthesis of dNTPs. Active RNR is a heterodimeric protein complex. In S phase cells, the mouse RNR consists of the R1 and the R2 proteins. The R1/R2 RNR-complex supplies the cell with dNTPs required for DNA replication. Outside S-phase or in non-proliferating cells RNR is composed of R1 and p53R2 proteins. The R1/p53R2 RNR-complex supplies cells with dNTPs required for mitochondrial DNA replication and for DNA repair. An undisturbed dNTP regulation is important since unbalanced dNTP pools results in DNA mutations and cell death. Since unbalanced pools are harmful to the cell, RNR activity is regulated at many levels. The aim of this thesis is to study how the mouse RNR genes are regulated at a transcriptional level. We have focused on the promoter regions of all three mouse RNR genes. Primer extension experiments show that the transcription start of the TATA-less p53R2 promoter colocalizes with an earlier unidentified initiator element (Inr-element). This element is similar to the known Inr-element in the mouse R1 promoter. Furthermore, functional studies of the R1 promoter revealed a putative E2F binding element. This result suggests that the S phase specific transcription of the R1 gene is regulated by a similar mechanism as the R2 promoter which contains an E2F binding site. Finally we have established a method to partially purify the transcription factor(s) binding the upstream activating region in the mouse R2 promoter by phosphocellulose chromatography and affinity purification using oligonucleotides immobilized on magnetic beads. This method will allow us to further study the transcription factors responsible for activating expression of the R2 protein. This method has a potential to be utilized as a general method when purifying unknown transcription factors.