Although reptiles are well known for their low energy requirements and high efficiencies of biomass conversion, few studies have addressed the ecology and energetics of species with very low activity levels. I investigated the ecology and energetics of two such reptilian groups: helodermatid lizards, and viperid snakes. I radiotracked Mexican beaded lizards and three rattlesnake species in their natural environments to determine their activity patterns, the time and energy they invest in activity, home ranges, thermal biology, habitat and other resource use, and behavior. I measured metabolic rates in the laboratory to determine rates of energy use. Beaded lizards had a mean home range of 21.6 ha, an activity peak at 1800 hrs and traveled, on average, 25.3 km during approximately 121 h of surface activity over the year. They had very low metabolic rates during rest. However, helodermatids had maximal rates of oxygen consumption that were among the highest of any lizard measured, a trait that may be adaptive during their intensive male-male agonistic behaviors. Males had significantly higher capacities for aerobic activity than did females. Heloderma horridum can fulfill its annual maintenance energy requirements with a quantity of prey equivalent to approximately 1.4 times its body mass. Rattlesnakes had a mean home range size of 4.6 hectares and traveled, on average, 12.4 km over approximately 95 hours of annual surface activity. Rattlesnakes spent considerable time inactive on the surface, whereas Heloderma spent the vast majority of their time resting in shelters. Heloderma and Crotalus had similar body temperature preferences for activity (around 30 C). Like Heloderma, Crotalus had very low rates of metabolism during rest. A 300-g rattlesnake had a standard metabolic rate only 40% that of other squamate reptiles of similar mass, and could fulfill its yearly maintenance energy requirements with a prey quantity equivalent to 0.93 x its body mass, which could be met with 2-3 large meals. After feeding, free-ranging rattlesnakes exhibited thermophilic responses that varied, in part, due to their reclusive behavior, and thermal constraints in the environment. The temperatures selected for digesting and activity are apparently similar in rattlesnakes.