The permeability of L-dopa across the blood-brain barrier was studied in twelve patients by means of the indicator dilution method. The extraction of [1-14C]L-dopa and [3-14C]L-dopa was 11.7% and 10.4%, respectively. After pretreatment with carbidopa the extraction was unchanged (13.5% and 11.6%). The permeability coefficient (P) was calculated to 0.9 x 10(-5) cm s-1.