Is DNA replication/ cell proliferation in vertebrates possible during anoxia? The oxygen dependence of ribonucleotide reductase (RNR) could lead to a stop in DNA synthesis, thereby making anoxic DNA replication impossible. We have studied this question in an anoxia tolerant vertebrate, the crucian carp (Carassius carassius), by examining 5'-bromo-2'-deoxyuridine (BrdU) incorporation and proliferating cell nuclear antigen (PCNA) levels in the gills, intestinal crypts and liver. We exposed crucian carp to 1 and 7 days of anoxia followed by 7 days of reoxygenation. There was a reduced incidence of S-phase cells (from 12.2% to 5.0%) in gills during anoxia, which coincided with a concomitant increase of G₀-cells. Anoxia also decreased the number of S-phase cells in intestine (from 8.1% to 1.8%). No change in the fraction of S-phase cells (~1%) in liver was found. Thus, new S-phase cells after 7 days of anoxia were present in all tissues revealing a considerable rate of DNA synthesis. Subsequently, the oxygen dependent subunit of crucian carp RNR (RNRR2) was cloned. We found no differences in amino acids involved in radical generation and availability of the iron centre compared to mouse, which could have explained reduced oxygen dependence. Further, the amount of RNRR2 mRNA in gills did not decrease throughout anoxia exposure. These results indicate that crucian carp is able to sustain some cell proliferation in anoxia possibly because RNRR2 retains its tyrosyl radical in anoxia, and that the replication machinery is still maintained. We have previously observed that hypoxia triggers a 7.5 fold increase of respiratory surface area in crucian carp. This response was not triggered in anoxia.