Nine young healthy postmen performed three progressive maximal exercise tests (on a bicycle ergometer and with and without a 10-kg shoulder bag on a treadmill) for which the least-squares regression equations of heart rate (HR) on oxygen consumption (VO) were established. HR and VO2 were then measured during five modes of simulated postal delivery: on level ground by foot (F), bicycle (B), and push-cart (C) and on stairs by foot (S/F) and push-cart (S/C). The respective work HR values were inserted into the three regression equations established for HR/VO, and VO was solved for. The estimated VO values tended to be systematically higher than the measured ones. The best regression proved to be that established for the treadmill plus mailbag; the regression coefficients between the measured and estimated VO2s were higher than 0.75, and the percentage differences between the means of the measured and estimated values were no more than 10% for F and B delivery. All the regressions yielded inaccurate estimations of VO for C, S/F and S/C delivery. It was concluded that HR is acceptable as an estimator of VO only when the test work closely resembles the muscle work in the job.