The final step in heme synthesis is catalyzed by the mitochondrial enzyme, ferrochelatase. Characterization of this enzyme has been complicated by a number of factors including the dependence of enzyme activity on lipids. Purification of ferrochelatase from rat and bovine sources has been achieved only relatively recently using blue Sepharose CL-6B chromatography. When 3,5-diethoxycarbonyl-1,4-dihydro-2,4,6-trimethylpyridine (DDC) is given to animals, it produces a hepatic porphyria resembling human variegate porphyria thus providing an experimental system in which to study this disease. DDC has been found to cause the accumulation of a green pigment, identified as N-methyl protoporphyrin IX (N-MePP), which is a potent inhibitor of ferrochelatase. The source of the N-methyl substituent of N-MePP was found to be the 4-methyl group of DDC. Considerable evidence indicates that the protoporphyrin IX moiety of N-MePP originates from the heme moiety of cytochrome P-450 and that DDC is a suicide substrate for this hemoprotein. Some studies suggest that cytochrome P-450 isozymes differ in their susceptibility to destruction by DDC and its 4-alkyl analogues. Griseofulvin has also been reported to inhibit hepatic ferrochelatase in rodents but not in the 17-day old chick embryo nor in hepatocyte culture systems. Thus, the mechanism by which griseofulvin produces an experimental porphyria in chick embryo liver cell culture is different from that for rodents.