Abstract: A parallel-series four-wheel-drive hydraulic circuit for a riding lawn mower. The lawn mower includes first and second hydraulic circuits. The first hydraulic circuit comprises a first hydraulic motor drivingly connected to a first wheel, a second hydraulic motor drivingly connected to a second wheel, a hydraulic pump connected in series with said first and second hydraulic motors, and a first fluid path allowing recirculation of hydraulic fluid through the second motor so that the speed of the second motor can increase relative to the speed of the first motor. Correspondingly, the speed of the second wheel can increase relative to the speed of the first wheel. A first conduit communicates between the first and second motors, a second conduit communicates between the second motor and the pump, and the fluid path communicates between the first and second conduits. The fluid path includes a check valve preventing reverse fluid flow through the fluid path from the first conduit to the second conduit.