The milk energy intakes and growth rates of suckling young at peak lactation of 62 mammalian species and subspecies, all measured using either the weigh–suckle–weigh, the isotope dilution or the isotope transfer method, were evaluated. The mean daily gross energy intakes (GEI) were as low as 12 kJ in a small rodent (mouse) to as high as 249 MJ in a phocid seal (hooded seal Cystophora cristata), while the daily growth rates at peak lactation ranged from 0.4 to 5.9 kg. Several allometric equations were calculated to explore the relationships between gross energy intake via milk and body weight (BW), growth rate and BW, as well as between gross energy intake via milk and growth rate. The results suggest that both GEI via milk and growth rates are proportional to BW to the power of 0.82. Accordingly, the metabolic weight of suckling mammalian young should be expressed as BW0.82. The predictive values of the calculated equations indicate that suckling young at peak lactation consume c. 883 kJ day−1 kg−1 BW0.82 and have growth rates of 32 g day−1 kg−1 BW0.82. However, large deviations for some species and few outliers were found. These equations could be used to predict values for species that have not been studied, provided that the BW falls within the range of weights used to derive the equations.