Energy cost is a major factor influencing the tolerable thermal load, particularly during exercise in the heat. However, no data exist on the metabolic cost of football practice, although a value of 35% of maximal aerobic capacity (VO(2)max) has been estimated. The energy cost and thermoregulatory response of offensive linemen (OL) was measured wearing different American football ensembles during a simulated half of football practice in the heat. Five collegiate offensive linemen (133 kg, 20% fat, 42 ml x kg(-1) x min(-1) maximal oxygen uptake) completed each of four 60-minute test sessions in an environmental chamber (28 degrees C, 55% relative humidity [RH]) wearing shorts (S), helmet (H), helmet and shoulder pads (HS), and full gear (FUL). Core temperature in the digestive tract (TGI) was obtained using an ingestible sensor. During simulated football drills (e.g., repetitions of drive blocking), exercise intensity ranged from 30 to 81% VO(2)max but averaged 55%VO(2)max (6.7 METS) overall. Blood lactate remained >5 mmol x L(-1), and heart rate (HR) averaged 79%HRmax. Equipment had a significant effect on %VO(2)max but only during recovery between drills with HS (61.4 +/- 3.7%) compared with H (53.3 +/- 6.9%) and S (40.1 +/- 8.5%). The TGI was higher (p < 0.05) with HS compared with H at several time-points after 30 minutes. Football practice for OL elicits a significantly higher overall metabolic cost (>6 METS, >50%VO(2)max) than assumed in previous studies. The addition of shoulder pads increases core temperature and energy cost, especially during recovery between active drills in unacclimatized linemen.