Co-protoporphyrin, like Co2+, produced a significant and persistent induction of hepatic ornithine decarboxylase (ODC) as well as its known inducing effect on heme oxygenase and the decreasing effects on drug-metabolizing enzymes. The induction of ODC and heme oxygenase by Co-protoporphyrin occurred dose-dependently with the lowest effective dose of 6.25 mumol/kg. Although Co-protoporphyrin produced similar effects on ODC and heme oxygenase to Co2+, there were differences in the mode of ODC induction. In particular, pretreatment with diethyl maleate failed to augment the induction of ODC by Co-protoporphyrin. Moreover, multiple administrations of Co2+, but not Co-protoporphyrin, caused super-additive induction of ODC to about 1000-fold over the controls. This super-additive induction of ODC by Co2+ was dependent on the doses and time intervals between two administrations. In parallel with a large induction of ODC evoked by two administrations of Co2+, hepatic putrescine content was increased markedly, while spermine content was decreased as compared to the control levels. Pretreatment with Co2+ led to super-additive induction of ODC by subsequent administration of the metal ion itself or diethyl maleate, but not by other ODC inducers, such as Co-protoporphyrin and thioacetamide, and not by subsequent partial hepatectomy. Under these experimental conditions, the magnitudes of heme oxygenase induction were similar. ODC induced by two doses of Co2+ was insensitive to exogenous putrescine, but sensitive to alpha-difluoromethylornithine and 1,3-diaminopropane. These findings add new insight into the effects of Co2+ and Co-protoporphyrin on hepatic polyamine metabolism; and the results suggest that the metal ion could cause extensive derangement of the ODC regulatory system in a manner different from the metalloporphyrin.