Metabolic rate was determined once a week in broilers from a commercial source, from 1 to 63 d of age. The equations relating minimal resting metabolic rate (oxygen consumption, ml/bird h, y) and body weight (W) were: males 45 to 497 g, y = 3.2 W0.882; 597 to 3000 g, y = 40.5 W0.483; females 45 to 514 g, y = 2.52 W0.881; 514 to 2500 g, y = 12.3 W0.627. The relationship between lower critical temperature (Tcl, degree C, y) and age (d, x) may be described by the following equations: chicks 1 to 21 d, y = 34.2 - 0.32 x; 14 to 63 d, y = 49.4 x -0.194. The relationship between Tcl and W may be described by one equation for both sexes between 100 and 3000 g, y = 62.15 W-0.135. The equations for Tcl and data for upper critical temperature (Tcu) could be used to obtain maximal performance from broilers, with reduced costs, by providing a suitable environment related to age or body weight.