Three groups of specialist nectar-feeders covering a continuous size range from insects, birds and bats have evolved the ability for hovering flight. Among birds and bats these groups generally comprise small species, suggesting a relationship between hovering ability and size. In this study we established the scaling relationship of hovering power with body mass for nectar-feeding glossophagine bats (Phyllostomidae). Employing both standard and fast-response respirometry, we determined rates of gas exchange in Hylonycteris underwoodi (7 g) and Choeronycteris mexicana (13-18 g) during hover-feeding flights at an artificial flower that served as a respirometric mask to estimate metabolic power input. The O2 uptake rate (VO2) in ml g-1 h-1 (and derived power input) was 27.3 (1.12 W or 160 W kg-1) in 7-g Hylonycteris and 27.3 (2.63 W or 160 W kg-1) in 16.5-g Choeronycteris and thus consistent with measurements in 11.9-g Glossophaga soricina (158 W kg-1, Winter 1998). VO2 at the onset of hovering was also used to estimate power during forward flight, because after a transition from level forward to hovering flight gas exchange rates initially still reflect forward flight rates. VO2 during short hovering events (< 1.5 s) was 19.0 ml g-1 h-1 (1.8 W) in 16-g Choeronycteris, which was not significantly different from a previous, indirect estimate of the cost of level forward flight (2.1 W, Winter and von Helversen 1998). Our estimates suggest that power input during hovering flight Ph(W) increased with body mass M (kg) within 13-18-g Choeronycteris (n = 4) as Ph = 3544 (+/- 2057 SE) M1.76 (+/- 0.21 SE) and between different glossophagine bat species (n = 3) as Ph = 128 (+/- 2.4 SE) M0.95 (+/- 0.034 SE). The slopes of three scaling functions for flight power (hovering, level forward flight at intermediate speed and submaximal flight power) indicate that: 1. The relationship between flight power to flight speed may change with body mass in the 6-30-g bats from a J- towards a U-shaped curve. 2. A metabolic constraint (hovering flight power equal maximal flight power) may influence the upper size limit of 30-35 g for this group of flower specialists. Mass-specific power input (W kg-1) during hovering flight appeared constant with regard to body size (for the mass ranges considered), but differed significantly (P < 0.001) between groups. Group means were 393 W kg-1 (sphingid moths), 261 W kg-1 (hummingbirds) and 159 W kg-1 (glossophagine bats). Thus, glossophagine bats expend the least metabolic power per unit of body mass supported during hovering flight. At a metabolic power input of 1.1 W a glossophagine bat can generate the lift forces necessary for balancing 7 g against gravitation, whereas a hummingbird can support 4 g and a sphingid moth only 3 g of body mass with the same amount of metabolic energy. These differences in power input were not fully explained by differences in induced power output estimated from Rankine-Froude momentum-jet theory.