Heme is essential for cell respiration, energy generation, and oxidative biotransformations. The latter function is exemplified by the oxidative metabolism of various endogenous and exogenous chemicals catalyzed by the heme protein cytochrome P-450. Recent studies have established that metal ions directly regulate cellular content of heme, and thus of heme proteins by controlling production of delta-aminolevulinate synthetase and heme oxygenase, the rate-limiting enzymes for heme synthesis and degradation, respectively. Metal ions also alter cellular content of glutathione. In excess amounts, metal ions greatly accelerate the turnover and degradation of heme and substantially impair the oxidative functions of cells--particularly those dependent on cytochrone P-450. As a result, the biological impact of chemicals which are detoxified or metabolically transformed by the P-450 system is greatly altered.