The Rio Grande rift, which is marked by a positive heat-flow anomaly in southern New Mexico, has been the subject of a gravity study based on 4,500 stations which cover a strip across New Mexico. The area consists of a series of basins and intervening ranges formed during Miocene time. This basin-and-range structure is strongly reflected in the Bouguer gravity anomalies, which range from −125 mgal over uplifts to −190 mgal over basins. Lineaments in trends of gravity anomalies are oblique to the predominant north-south trend of the rift and suggest that, in detail, the crust broke upon fractures oblique to the large-scale north-south trend. Thicknesses of Cenozoic sediments, determined from gravity measurements, range from 2 to 3 km in basins. The gravity effect of sediments is removed by stripping, and a broad +30-mgal gravity anomaly is located over the rift. Regional and residual Bouguer gravity anomaly maps have been constructed. The source of the 30-mgal gravity high is interpreted to be a shallow slab of basalt or a deep upwarp of the mantle that results in crustal attenuation. The low-velocity zone may project up toward the base of the crust under the Basin and Range province. Experiments and the observed fault pattern suggest an extensional origin for the Rio Grande rift fracture system.