Studies with synthetic metal-porphyrin complexes in which the central iron atom of heme is replaced by other elements indicate that those heme analogues that cannot be enzymatically degraded to bile pigments possess novel biological properties that may have considerable clinical as well as experimental value. Such studies have revealed the important role that the central metal atom plays in determining the physiological and pharmacological properties of metal-porphyrin complexes; and they have demonstrated that the form in which animals and humans are exposed to trace metals, i.e., inorganic, organified, porphyrin-chelated, etc., can be of great importance in determining the biological responses that such elements elicit, especially with respect to actions on heme synthesis and degradation and cytochrome P-450 formation and function. Study of the biological properties of synthetic metalloporphyrins represents a potentially fruitful area of research and the results may have significant value for basic as well as clinical disciplines.