Summary1. The pattern of sulphate acceptors present in the urine of phenylketonurics and control cases was visualized by chromato‐autoradiographic methods, after treatment of urine specimens in a sulphur ‐ylating rat‐liver system in the presence of S35‐labelled sulphate.2. The sulphate of o‐HPAA formed during this procedure from o‐HPAA excreted in the urine of phenylketonurics could be demonstrated as a well‐defined spot (pk 1) on the autoradiograms in all untreated cases.3. Another sulphate acceptor, revealed after acid hydrolysis, was shown to be identical with the hydantoin formed from urea and p‐HPPA during hydrolysis. The concentration of the ester sulphate of this hydantoin (pk 2) probably reflects the amount of p‐HPPA excreted in phen‐ylketonuria. This compound did not vanish from the urine specimens of patients given a low‐phenylalanine diet for periods of up to 13 months. It was, however, absent in one case treated for 29 months. The significance of these findings, as well as the possibilities of the method applied, are discusspd.