Graphene/poly(o-phenylenediamine) (GN/PoPD) modified disposable pencil graphite electrode (PGE) was prepared using one-pot electropolymerization and the use of this novel and practical electrode in dopamine (DA) sensing was investigated in the present study. This is the first attempt to prepare this nanocomposite in a one step, very easy and low-cost electropolymerization procedure onto pencil graphite electrode and then to use the coated surface for dopamine detection in untreated biological fluids. Electrochemical performance of the electrode was compared with the poly(o-phenylenediamine) modified, graphene modified and unmodified PGEs. Improved electrochemical responses were obtained in the case of graphene/poly(o-phenylenediamine) modified one. The electrode responded in a wide linear range of 1.0 x 10(-3) mu M to 150.0 mu M with a very low detection limit of 0.16 nM for dopamine. It exhibited highly sensitive and selective results in the presence of ascorbic acid (AA) and uric acid (UA). The synergestic effects between poly(o-phenylenediamine) and graphene exhibited excellent electrocatalytic activity, selectivity and sensitivity toward the oxidation of ascorbic acid, dopamine and uric acid, resulting the simultaneous detection of these molecules. The biosensor applied to untreated serum samples resulted good recovery values and it differentiated ascorbic acid, dopamine and uric acid in a high sensitivity and selectivity. The peak separations between ascorbic acid, dopamine and uric acid were 148 mV and 133 mV, respectively.