The pharmacokinetics of levodopa differs when it is combined with benserazide or carbidopa. Peak dopa levels are higher, occur sooner, but decline more rapidly with benserazide. Although many patients respond better to one drug than the other, we sought to exploit the differences in pharmacokinetics by giving both drugs to the same patient. Benserazide was combined with carbidopa in 38 patients who were experiencing a diminished response to carbidopa, including 22 patients with diurnal oscillations in performance, "wearing off" or on-off phenomena. Previous attempts to change the dose, sequence, or ratio of levodopa to carbidopa in these patients had been unrewarding. Ten of the patients improved on the combination of benserazide and carbidopa, with a 30% decline in disability. The mean dose of levodopa:carbidopa before benserazide was 910:100 (9 to 1 ratio); the mean dose of levodopa:benserazide was 355:90 (4 to 1 ratio). The mean dose of levodopa:carbidopa + benserazide was 925:155 (6 to 1 ratio). The combination of carbidopa with benserazide is useful in some parkinsonian patients.