Rats received 10 mg. of L-or D-penicillamine daily over a period of three weeks. 1. The excretion of xanthurenic acid after loading with 40 mg. tryptophan increased from 0.2 mg. in the control animals to 5.7 mg. after L-penicillamine and 0.6 mg. after D-penicillamine, 2. The aspartate-aminotransferase activity in the liver of L-penicillarnine-treated animals was decreased by 20%, while D-penicillamine had no effect. 3. The alanine-aminotransferase activity was decreased by 50% in the liver after treatment with L-penicillamine, and by 20% after Dpenicillamine.