The effect of doubling carbidopa intake on single dose bioavailability of L-Dopa was examined in five parkinsonian patients. Increasing carbidopa from a mean of 145 to 290 mg/day caused a mean increase in peak plasma Dopa concentrations from 1,893 +/- 476 to 2,733 +/- 607 ng/ml and area under the Dopa plasma concentration versus time curve of 24.6 +/- 10.0%, as well as a decrease in the time to peak plasma concentration to 0.7 +/- 0.2 versus 1.2 +/- 0.3 h. Increasing the within-dose ratio of carbidopa to L-Dopa, even in patients receiving "maximally effective" doses of carbidopa, further increases bioavailability of L-Dopa, presumably by inhibiting "first pass' metabolism in the gut.