The dark matter distribution of the Milky Way remains among the major unsolved problems about our home galaxy. The masses of other spiral galaxies can be determined from their rotation curves through long-slit spectroscopy. But for the Milky Way obtaining the complete rotation curve is a more complex problem. By measuring the 3-dimensional motions of tracer objects the rotation curve and Galactic mass distribution can be derived, even outside the solar circle where HI tangent point analysis is not possible. We present the first findings from a project to measure the motions of open clusters, both inside and outside the solar circle. From a nearly uniform sample of spectroscopic data for large numbers of stars in over 50 open clusters in the third and fourth Galactic quadrants, we derive the speed of Galactic rotation at the solar circle as Theta0 = 214+6-9 km/s. Future work will include clusters in the other Galactic quadrants and analysis of the local rotation curve.