A method is described for the colorimetric estimation of 6-hydroxyskatole sulfate excretion in urine. The assay procedure involves the extraction of 6-hydroxyskatole sulfate from urine with butanol. The butanol was removed in vacuo and the residue taken up in water, treated with a sulfatase preparation and the free 6-hydroxyskatole determined by reaction with diazotized sulfanilic acid under acid conditions. The assay was run on 46 chronic schizophrenic patients and 36 control subjects. Significant differences were found in the excretion of 6-hydroxyskatole sulfate. The significance of this finding with regard to schizophrenia requires further investigation.