Two isozyme forms of arginase are found in the rat. All arginases are metalloenzymes which require manganese for activity. Many arginases are activated by cobalt and nickel ions and inhibited by heavy metal ions. The purpose of this study was to compare the effect of other heavy metal ions on the rat liver isozyme (arginase I) and the rat kidney isozyme (arginase II). The activation and inhibition of arginase I and II by metal ions were different. However, both isozymes were strongly inhibited by cupric and mercuric ions. The inhibition of arginase I by cupric and mercuric ions was increased greatly by preincubation of the enzyme with the metal ions. However, preincubation of arginase II by cupric and mercuric ions had little effect on the inhibition of the enzyme. Under certain conditions the kinetics of the inhibition of both arginases I and II by cupric and mercuric ions was nonlinear allosteric.