New heat-flow data obtained in northeastern New Mexico and southeastern Colorado show three regional trends: (1) A broad heat-flow anomaly associated with the southern Rocky Mountains contrasts with a narrow heat-flow anomaly between lat 35.5° and 34°N, apparently associated only with the Rio Grande rift. (2) The high heat-flow anomaly apparently associated with the southern Rocky Mountains extends 200 to 300 km onto the Great Plains of northeastern New Mexico and southeastern Colorado. (3) Areas of extensive volcanic activity do not necessarily have high heat flow. In addition, measurements of crustal radioactivity in the vicinity of the Rio Grande rift suggest that the radioactive heat generation contributes uniformly to the surface heat flow. This implies that the heat-flow anomaly observed along the Rio Grande rift is caused by tectonic and volcanic sources and not by anomalously high crustal radioactivity.