1. The P450 gene superfamily is presently known to contain more than 78 members, divided into 14 families. 2. The superfamily has undergone divergent evolution, and the ancestral gene is probably more than 2 billion years old. 3. The recent 'burst' in new P450 genes, particularly in the II family during the past 800 million years, appears to be the result of 'animal-plant warfare'. 4. Due to the presence or absence of a particular P450 gene in one species but not the other, it may not be correct to extrapolate toxicity or cancer data from rodent to human. 5. Increases in the P450 gene product (enzyme induction) almost always reflect an elevated rate in gene transcription, although there are several exceptions. 6. The mechanisms of P450 gene regulation (induction) by classes of inducers might become better understood through the comparison of different phyla that differ in response to a particular class of inducers. 7. Amongst several carefully selected phyla, delineation between which electron donor (presence of Fe2S2 protein or NADPH-P450 oxidoreductase, or both) interacts with P450 may provide valuable information about the evolution of eukaryotes from prokaryotes.