1. The present work has examined the effects of short- (30 min) and long-term (14 h) exposure to cyclosporine A (CsA) on the uptake of L-DOPA, its decarboxylation to dopamine and the cellular extrusion of taken up L-DOPA and of newly-formed amine in monolayers of LLC-PK1 cells. 2. In the presence of benserazide (50 microM), L-DOPA was rapidly accumulated in LLC-PK1 cells (cultured in collagen-treated plastic) attaining equilibrium at 30 min of incubation. Non-linear analysis of the saturation curves revealed a Km of 113+/-16 microM and a Vmax of 5581+/-297 pmol mg(-1) protein 6 min(-1). 3. In the absence of benserazide, LLC-PK1 cells incubated with increasing concentrations of L-DOPA (10 to 500 microM) for 6 min accumulate newly-formed dopamine by a saturable process with apparent Km and Vmax values of 31+/-6 microM and 1793+/-91 pmol mg(-1) protein 6 min(-1), respectively. The fractional outflow of newly-formed dopamine was found to be 20%. Up to 200 microM of intracellular newly-formed dopamine, the outward transfer of the amine was found to be a non-saturable process. 4. Short-term exposure to CsA (0.3, 1.0 and 3.0 microg ml(-1)) was found not to change the intracellular concentrations of newly-formed dopamine, but increased the levels of dopamine in the incubation medium (143% to 224% increase) and the total amount of dopamine formed (31% to 59% increase). Long-term exposure to CsA (0.03 to 3.0 microg ml(-1)) reduced the total amount of dopamine (15% to 39% reduction) and the intracellular levels of the amine (11% to 56% reduction), without changing dopamine levels in the incubation medium. Both short- and long-term exposure to CsA resulted in a concentration-dependent increase in the fractional outflow of newly-formed dopamine. 5. Short-term exposure to CsA (3.0 microg ml(-1)) reduced the apical extrusion of intracellular L-DOPA by 15% (P<0.05), whereas long-term exposure to CsA reverted this effect and decreased its intracellular availability (15% reduction; P<0.05). 6. Detection of P-glycoprotein activity was carried out by measuring verapamil- or UIC2-sensitive rhodamine 123 accumulation. Both UIC2 (3 microg ml(-1)) and verapamil (25 microM) significantly increased the accumulation of rhodamine 123 in LLC-PK1 cells. A 30 min exposure to CsA was found not to affect the accumulation of rhodamine 123 in the presence of verapamil (25 microM), whereas a 14 h exposure to CsA was found to reduce the accumulation of rhodamine 123. 7. In conclusion, the increase and the reduction in the formation of dopamine after short- and long-term exposure to CsA, respectively, correlate with the effects of the immunosuppressant on the apical cell extrusion of taken up L-DOPA, suggesting the involvement of P-glycoprotein. The effects of CsA on the fractional outflow of newly-formed dopamine appear to be mediated by a different mechanism.