We examined whether heart rate (HR) could be used to estimate energy expenditure (EE) in cattle. Six Hereford heifers (345 +/- 10.8 kg BW) 12 mo of age were implanted with HR radio transmitters and maintained in individual pens under the following treatments: 1) shade or sun exposure, 2) high- or low-energy diet, and 3) feeding in morning or afternoon. The HR of animals was measured every .5 h during 3 mo; measurements of oxygen consumption and HR were made simultaneously in the morning and in the afternoon while animals were resting and exercising. Average daily HR (52 +/- 4 beats/min) and average daily EE (380 +/- 9 kJ/kg(.75)) in animals on the low-energy diet were less than values in animals on the high-energy diet (94 +/- 4 beats/min and 653 +/- 9 kJ/ kg(.75), respectively). For each animal and within each diet, linear regressions best described the relationship between HR and EE in resting animals, whereas quadratic regressions best described this relationship for exercising animals. The quadratic equation for the exercising animals could also be used for resting animals. In addition, a constant value of EE per heart beat (EE pulse) for each individual resting animal was found and gave accurate estimations. This method was convenient because 1) no exercise equipment was needed to generate the regression equations and 2) EE pulse was less affected by diet than was EE estimated by regression equations. We conclude that HR, a relatively easy measurement, can be useful and accurate in estimating EE. To increase the accuracy of the estimation of EE by HR, the relationship of HR to EE should be established for each animal. In addition, the nutritional regimen for the animal in which EE is estimated should be used for the animal in establishing the relationship.