The aim of this study was to characterize untrained Nandi boys (mean age 16.6 years) from a town (n = 11) and from a rural area (n = 19) in western Kenya (altitude approximately 2000 m.a.s.l.) in regard to their body dimensions, oxygen uptake and physical activity level. The town boys had a mean maximal oxygen uptake (VO(2 max)) of 50 (range: 45-60) mL kg(-1) min(-1), whereas the village boys reached a value of 55 (37-63) mL kg(-1) min(-1) ( p<0.01) in VO(2 max). The running economy, determined as the oxygen cost at a given running speed, was 221 mL kg(-1) km(-1) (597 mL kg(-0.75) km(-1)) for town as well as for village boys. The body mass index (BMI) was very low for town as well as for village boys (18.6 vs 18.4 kg m(-2)). The daily mean time spent working in the field during secondary school and doing sports were significantly higher in village boys compared to town boys (working in the field: 44.2 (0-128) vs 1.3 (0-11) min, p<0.01; sports: 32.0 (11-72) vs 12.8 (0-35) min, p<0.01, respectively). A positive correlation between the daily time spent doing sports and VO(2 max) was found when pooling the data from the town and the village boys (R = 0.55, p<0.01). It is concluded that the body dimensions of adolescent Nandi town and village boys corresponds well with findings in Kenyan elite runners. They are very slender with relatively long legs. In addition, the VO(2 max) of the village boys was higher than that of the town boys, which is probably due to a higher physical activity level of the village boys during secondary school.