1. The effects of phenylhydroxylamine, and o- and p-aminophenol were studied in the Japanese quail. 2. Symptoms normally observed in aniline-treated birds were seen in quail after phenylhydroxylamine dosage at > 10 mg/kg. Aminophenols (up to 50 mg/kg) did not give these symptoms. 3. Injection of phenylhydroxylamine (50 mg/kg) resulted in formation of 70% ferrihaemoglobin after 5 min, following which a rapid reduction of ferrihaemoglobin was observed. 4. Phenylhydroxylamine reached highest blood concn. of 0.2 mumol/ml after 5 min. Phenylhydroxylamine was reduced to aniline within 5 min. 5. The effects of aniline in vivo are most probably due to O2 shortage caused by ferrihaemoglobin formation.