Seven patients with Parkinsonism previously on l-dopa were placed on a regimen of l-dopa and alpha methyl dopa hydrazine (a dopa decarboxylase inhibitor). Two of these patients had previously shown marked clinical deterioration of the l-dopa improvement when given pyridoxine. None of the seven patients receiving alpha methyl dopa hydrazine demonstrated any change in their condition when given pyridoxine. The failure of vitamin B(6) to reverse the clinical effect of l-dopa in patients receiving both l-dopa and a peripheral dopa decarboxylase inhibitor suggests that reversal of the l-dopa effect induced by vitamin B(6) is due to increasing the activity of the enzyme dopa decarboxylase outside the central nervous system.