The Madison Limestone forms a highly productive aquifer in the Powder River basin, Wyoming. The Madison in the study area is a highly fractured limestone, 90 to 200 m thick, with significant percentages of dolomite. Study of Madison water quality in Weston and Crook Counties, Wyoming indicated rapid flow through the aquifer, little or no intrusions from adjacent strata, and absence of evaporite deposits throughout the study area. Ground-water ages near the Black Hills monocline were calculated by using tritium data and flow-net analysis. Both methods predicted water ages of less than 210 years at Newcastle and Osage, Wyoming. Calculated seepage velocities were on the order of 0.8 m/day. Geochemical and hydrodynamic data suggested that the Black Hills monocline is a major feature in the Madison flow system. Changes in the water quality and potentiometric gradient perpendicular to the axis of the monocline indicated that little water is traveling westward from the recharge areas into the deep centeral basin, a high-transmissivity zone exists along the axis of the monocline, and the high-transmissivity zone along the monocline may be the major pathway for Madison ground water flowing out of the eastern Powder River basin.