Levels of 5-hydroxyindoleacetic acid, indoleacetic acid and total indoles were determined in the urine of 23 patients with systemic scleroderma and 7 patients with cutaneous scleroderma, before and after peroral loading with L-tryptophan (0-1 g/kg body weight). Before loading, 5-hydroxyindoleacetic acid levels were normal in nearly all cases of systemic scleroderma as well as of cutaneous scleroderma; however after loading, in nearly one half of cases there was no normal increase of this metabolite. These results suggest impaired transformation of serotonin into 5-hydroxyindoleacetic acid. A disproportionately high ratio of total indoles to indoleacetic acid suggests the presence of excess of tryptamine. The results of the study may indicate that in scleroderma metabolism of biogenic amines derived from tryptophan is abnormal, probably as a result of impaired activity of monoamine oxidase.