Sheep are grazers and goats are intermediate feeders. By employing O-2 consumption and heart rate measurements, resting metabolic rate (RMR) and field metabolic rate (FMR) were determined in four male fat-tailed Awassi sheep (44.0 +/- 3.94) and four male Baladi goats (35.5 +/- 5.42 kg) that were co-grazing natural pasture in the Negev Desert. There were 67.7 +/- 3.75 g DM/m(2) of herbaceous vegetation biomass, which was rapidly becoming senescent and more fibrous. We hypothesized that FMR of these desert-adapted ruminants would be relatively low when compared to other sheep and goat breeds, as animals in arid areas tend to have low metabolic rates. Both sheep (n = 6) and goats (n = 6) foraged 71% of the allotted 11 h free-pasture period; however, sheep grazed more than goats (P < 0.001); whereas goats browsed more than sheep (P < 0.001). RMR was higher (P = 0.007) in sheep than in goats (529 +/- 23.5 v. 474 +/- 25.4 kJ/kg(0.75) BW/d), but FMR did not differ between species (618 +/- 55.7 v. 613 +/- 115.2 kJ/kg(0.75) BW/d). In addition, the cost of activities, as a proportion of FMR, did not differ between sheep and goats; FMR increased by 89 kJ/kg(0.75) BW/d or 17% in sheep and by 138 kJ/kg(0.75) BW/d or 29% in goats. In comparing FMRs of sheep and goats in this study with these species in other studies, differences were inconsistent and, therefore, our hypothesis was not supported.