Activity of heme synthesis enzymes in newborn August and Wistar rats was studied after acute hypoxic hypoxia. Daily production of erythrocytes and activities of aminolevulinate synthase, aminolevulinate dehydratase, and heme synthetase were measured in the bone marrow (15-30 min after birth and on days 1 and 3 of life) and liver (day 3 after birth). Hypoxia was followed by a decrease in activity of heme synthesis enzymes in the liver (especially in August rats) and reduction of the daily erythrocyte production (especially in Wistar rats). Our results suggest that the response of heme synthesis enzymes to hypoxic exposure in newborn rats is genetically determined. The observed changes are more pronounced in Wistar rats.