In view of studies showing that not only nitric oxide synthase (NOS) activity but arginase activity is induced in rodent macrophages by lipopolysaccharide (LPS), the objective of this study was to investigate the co-induction of these two enzymes and to ascertain whether common mechanisms are involved. RAW 264.7 cells were activated by 2 micrograms LPS/ml and incubated for up to 48 hr. Inducible NOS (iNOS) and inducible arginase II (AII) activities were monitored, respectively, by measuring NO2-/NO3- accumulation in cell culture media and formation of urea (as CO2) from L-arginine by cell lysates. AII activity increased linearly up to at least 48 hr, whereas NO2-/NO3- formation reached a plateau well before 48 hr. Immunoprecipitation experiments revealed that AII accounted for 90-100% of arginase activity in LPS-activated macrophages. The inhibitor of NF-kappa B activation, pyrrolidine dithiocarbamate, inhibited the induction of iNOS but not AII. Moreover, whereas IFN-gamma caused iNOS induction, AII induction was nearly abolished by IFN-gamma, perhaps by inhibiting transcription of the AII gene. These observations indicate that co-induction of iNOS and AII occurs by distinct transcriptional mechanisms, AII induction could diminish NO production by decreasing L-arginine availability, and IFN-gamma can prevent AII induction.