The transport of dopamine from samples in which it is in the cationic form across a cation-exchange membrane into a receiver electrolyte occurred against its concentration gradient under two conditions. With receiver electrolytes at pH values below the pKA1 of dopamine, the transport was by Donnan dialysis. With the pH above the pKA1, the neutralization of protonated dopamine at the membrane-receiver interface sustained the diffusion gradient of the dopamine cation across the membrane phase, thereby allowing the analytical concentration of this species to increase above that of its concentration in the sample. When 0.25 M RbCl, 0.5 mM LaCl3 mixtures in the pH range 3.0-7.0 comprised the receivers, preconcentration factors of 20 were achieved with a 15-min dialysis across a tubular cation-exchange membrane. Under identical conditions except with a pH 10 receiver, the preconcentration factor was 22. The former condition results in Donnan dialysis whereas the transport mechanism at pH 10 is sustained passive diffusion. In contrast to Donnan dialysis, transport rates under sustained passive diffusion conditions are independent of ionic strength over a wide range; for example they are constant for samples containing up to 0.17 M KH2PO4 at pH 4.6.