The relative aerobic strain (RAS) of nonmotorized mail delivery was assessed in 54 Finnish mail carriers who represented both sexes, the entire workage range and both downtown and suburban delivery districts. The mean RAS of the entire delivery time was 55% of the maximal oxygen uptake (ml/kg-min). It was higher for women than for men, and higher for suburban than for downtown delivery. The RAS tended to increase systematically with age after the age of 50. The work strain of mail carriers of over 50 years of age, especially of older women carriers in suburban areas, was concluded to be high enough to lead to possible excessive strain on the workers.