The effects of in vivo administration of 6 compounds on the activity of delta-aminolevulinic acid (ALA) synthetase and heme oxygenase were determined. The order of decreasing potency in reducing ALA synthetase activity was heme, bilirubin, protoporphyrin IX, bilirubin dimethyl ester, CoCl2 and FeCl3. The chelating agents EDTA and deferoxamine did not prevent heme's repression of ALA synthetase or induction of heme oxygenase activity. The dose response, time course, enzyme subcellular distribution and chelation antagonism studies all suggest that heme itself, and not iron, regulates the rate limiting enzymatic steps of rat hepatic heme synthesis and degradation.