Wild type rat liver arginase contains two tryptophans, one partially exposed and one partially buried in the hydrophobic region. We measured the tryptophan fluorescence lifetime in wild-type arginase, recombinant arginase and two mutants each containing only one tryptophan. Energy transfer to Terbium atoms was observed under steady state conditions, however, no quenching of the emission was observed in the time resolved fluorescence emission experiments.