The diurnal cycle of metabolic rate (MR, J/g.h, based on V-o2 and V-CO2) was measured in 14 Griffon Vultures (Gyps fulvus), two Hooded Vultures (Necrosyrtes monachus) and one White-backed Vulture (Gyps africanus) at different ambient temperatures (-7 to +34 degreesC). In (so far) three Griffon Vultures the heart rate (HR) and body temperature (Tb) were measured by telemetry, simultaneously with MR. The three vulture species show very similar physiological mechanisms. In all cases measured MR is significantly below allometrically expected values (G. fulvus -46 %, N. monachus -24 %, G. africanus - 6 %). Fasting for 4 days results in an additional MR reduction of up to 27-35 %. There is a very small change of MR with ambient temperature (Ta). Therefore no obvious thermoneutral zone was observed in the broad Ta-range tested (see above). Ta-independent MR is the largest yet measured in birds. Thermal conductance (TC-wet) lies dramatically below expected values (-21.5 to -53.7 %), and is extensively used to control heat loss. All these special MR-strategies save energy expenditure. Tb of resting griffons shows a clear sinusoidal diurnal rhythm. The average Tb during the night has a mean value of 37.7 +/- 0.49 degreesC; the mean daytime value is 38.9 +/- 0.25 degreesC; the total average is 1.3/1.1 degreesC below expected values for Falconiformes, an effect significantly reducing energy expenditure. At Ta higher than about 25-30 degreesC, Tb increases significantly with increasing Ta, whereas MR does not vary significantly. Thus, the MR-independent variation of Tb may function as an additional and very effective mechanism for saving energy.