The effects of actinomycin D and cycloheximide on the original and CoCl2-induced delta-aminolevulinate-synthase and heme-oxygenase activities in rat liver were investigated. It was shown that 1.5 hours after CoCl2 administration the delta-aminolevulinate-synthase activity diminishes, then increases up to the original level within the subsequent 4.5 hours, showing a further increase thereafter. The heme-oxygenase activity does not change within 1,5 hours and is then increased. Actinomycin D and cycloheximide suppress the increase in the CoCl2-induced heme-oxygenase activity, whereas that of the delta-aminolevulinate-synthase activity is blocked only by cycloheximide. Hence, the increase in the CoCl2-induced delta-aminolevulinate-synthase activity in the liver is a result of activation of translation. The degree of tryptophan pyrrolase saturation with heme decreases already by the 6th hour, whereas the level of heme in liver mitochondria and microsomes decreases only by the 15th hour following CoCl2 injection. The heme content in the liver shows a further decrease irrespective of the increase in the delta-aminolevulinate synthase activity induced by CoCl2. It may be concluded that under the given experimental conditions this enzyme is not a rate-limiting step in the terminal reaction of heme biosynthesis.