The deoxyribonucleotides required for DNA synthesis are provided by reduction of ribonucleotides. Our studies aim at an understanding of the interplay between the processes involved in the production of deoxyribonucleotides and in their consumption. The enzyme system reducing ribonucleotides has been characterized in some detail, and its regulatory aspects were investigated with pure enzymes. Knowledge gained in this way made it possible to interfere specifically with the activity of ribonucleotide reductase in intact cells and to study effects on DNA synthesis. In this connection, the replication of polyoma DNA was used as a model that allowed dissection of DNA synthesis into different steps involved in the initiation and elongation processes.