Conclusions1. When the plasma para-aminohippuric acid concentration was maintained at or above 10 mg/100 ml a 3 to 6 fold elevation in penicillin plasma concentration was obtained except in two instances. 2. When the para-aminohippuric acid plasma levels were allowed to fall below 10 mg/100 ml there was no effect of that compound on the plasma concentration of penicillin. 3. The para-aminohippuric acid was non-toxic in the dosage given even when administered over a period of 8 days.