In situ hybridization was used to quantitate the cells that produce erythropoietin (EP) in the renal cortices of mice with varying severities of acute anemia and of mice recovering from severe, acute anemia. The number of EP-producing cells in the renal cortex increased in an exponential manner as hematocrit was decreased. Individual EP- producing cells had very similar densities of silver grains in autoradiograms regardless of whether they were from normal mice or from slightly, moderately or severely anemic animals. With increasingly severe anemia, total renal EP mRNA levels and serum EP concentrations showed increases that correlated with the number of renal EP-producing cells. These results indicate that as mice become more anemic, additional cells are recruited to produce EP rather than the cells already producing EP being stimulated to increase their individual production. In mildly and moderately anemic animals, small clusters of EP-producing cells were found in the inner cortex with large areas of cortex containing no EP-producing cells. In severely anemic mice, EP- producing cells were found throughout the inner cortex with only a very few found scattered in the outer cortex and outer medulla. The data indicate that only a subset of total renal interstitial cells produce EP. During recovery from severe, acute anemia, the numbers of EP- producing cells decreased exponentially as hematocrits rose and correlated with decreases in total renal EP mRNA and serum EP concentrations. These results suggest that following an acute blood loss and during the recovery from a blood loss, the capacity to deliver oxygen, as represented by hematocrit, is the major regulator of EP production.