To investigate the relation between "on-off" fluctuations in symptomatology and bioavailability of dopa in patients with Parkinson's disease, five Parkinsonian patients with pronounced "on-off" symptoms were studied. Continuously during the study the degree of disability in the patients was registered. Every one hour, and in addition, whenever there was a change from "on" to "off" or vice versa, a blood sample was collected for dopa determination. Since dopa is transported from plasma into the brain by a saturable carrier for which it has to compete with endogenous large neutral amino acids (LNAA), the concentrations of these competitors were measured too. In four of the patients there were considerable oscillations in the plasma dopa concentration during the day; in one of these patients the highest value was as much as 12 times higher than the lowest value. These dramatic fluctuations in the absolute concentration of dopa in plasma had a major influence on the relative dopa concentrations (calculated as the ratio dopa/sum of LNAA) as the fluctuations in the concentrations of LNAA in plasma were much less pronounced. Consequently, the absolute and the relative concentrations of dopa in plasma were highly parallelled. In four of the five patients "on"-periods began within one hour after a peak in the concentration of dopa in plasma and in the fifth patient five out of seven "on"-periods were preceded by a rise in plasma dopa concentration within the same time interval. From the present data it could be concluded that the "on-off" phenomenon in Parkinson's disease, at least partly, is due to oscillations in the concentration of dopa in plasma. A reduction in the variations of the concentration of dopa in plasma seems to be necessary to overcome the "on-off" problem. The introduction of a slow release preparation of dopa is therefore urgently warranted. The concentration of LNAA in plasma must, however, also be considered in this context.