5-Hydroxytryptophan has been found to be beneficial when administered alone or in combination with extracerebral aromatic amino acid decarboxylase inhibitors for therapeutic purposes in various disorders in which myoclonus is prominent. In five subjects the effect of decarboxylase inhibitors on the accumulation and elimination of 5-hydroxytryptophan in plasma was studied. The plasma concentrations of 5-hydroxytryptophan were increased about ten fold by pretreatment with the decarboxylase inhibitors, carbidopa and benserazide. Half-lives of 2.2--3.0 hrs. were obtained following oral administration of a single dose of 5-hydroxytryptophan with or without pretreatment. In one subject the half-life of 5-hydroxytryptophan in plasma increased to 5.5 hrs. during long-term treatment with carbidopa.