Alpha-synuclein is intimately involved in the pathogenesis of Parkinson’s disease, and has been implicated in the regulation of synthesis, release and reuptake of dopamine. However, mice lacking members of the synuclein family have been reported to display no overt behavioural phenotype. This may be a result of compensatory upregulation of other synucleins during development. Here we report on behaviour and dopamine synapse function of alpha-synuclein null, gamma-synuclein null and alpha-gamma-synuclein double-null knockout mice. Double-null mice were hyperactive in a novel environment and alternated at a lower rate in a T-maze spontaneous alternation task, a phenotype reminiscent of mice expressing reduced levels of the dopamine transporter. To investigate a possible hyperdopaminergic phenotype in alpha-gamma-synuclein double-null mice, we used fast-scan cyclic voltammetry at carbon-fiber microelectrodes to assess dopamine release and reuptake in striatal slices from wild-type, alpha-null, gamma-null and double-null mice in real time. Double-null mice were found to have a two-fold increase in the extracellular concentration of dopamine detected after discrete electrical stimuli in the striatum. By measuring the rate of reuptake of dopamine and tissue dopamine content in these animals, we showed that the observed increase in size of striatal dopamine transients was not attributable to a decrease in reuptake of dopamine via the dopamine transporter, and can not be attributed to an increase in tissue dopamine levels in the striatum. Rather, we propose that loss of both alpha and gamma-synuclein causes an increase in release probability from dopaminergic synapses.