THE MEN, AGED 24 THROUGH 59 YEARS, HAD SENTENCE LENGTHS RANGING FROM 12 MONTHS TO 30 MONTHS. ALL FELL WITHIN NORMAL HEIGHT AND WEIGHT RANGES. VENOUS BLOOD SAMPLES FROM BOTH GROUPS WERE TAKEN INTO LITHIUM HEPARIN TUBES, AND PLASMA WAS SEPARATED AND STORED AT 15 DEGREES BELOW ZERO CENTIGRADE UNTIL ANALYSIS. THE MASS-FRAGMENTOGRAPHIC ASSAY PROCEDURE WAS USED TO MEASURE PLASMA FREE AND CONJUGATED PHENYLACETIC ACID; ANALYSES WERE CARRIED OUT IN BLIND CONDITIONS. PLASMA CONCENTRATIONS OF FREE AND CONJUGATED PHENYLACETIC ACID WERE HIGHER IN THE 10 PRISONERS CONVICTED OF VIOLENT CRIMES AND SERVING LONG SENTENCES THAN IN THE MATCHED CONTROLS. SINCE AMPHETAMINE, A COMPOUND CLOSELY RELATED TO PHENYLETHLAMINE, CAN REDUCE AGGRESSIVENESS IN SOME VIOLENT SUBJECTS, THE INCREASE IN PHENYLETHYLAMINE PRODUCTION MAY BE AN ATTEMPT BY THE BODY TO COMPENSATE FOR THE UNKNOWN DERANGEMENT OF FUNCTION RESPONSIBLE FOR INCREASED AGGRESSION. REFERENCES AND TABULAR DATA ARE PROVIDED. (DAG)