Maximal O2 consumption (VO2max) and energy cost of running per unit distance (C) were determined on the treadmill in 36 male amateur runners (17 to 52 years) who had taken part in a marathon (42.195 km) or semi-marathon (21 km), their performance times varying from 1.49 to 226 and from 84 to 131 min, respectively. VO2max was significantly (2p less than 0.001) greater in the marathon runners (60.6 vs 52.1 ml . kg-1 . min-1) while C was the same in both groups (0.179 +/- 0.017, S.D., mlO2 . kg-1 . m-1 above resting), and independent of treadmill speed. It can be shown that the maximal theoretical speed in endurance running (vEND) is set by VO2max, its maximal sustainable fraction (F), and C, as described by: vEND = F . VO2max . C-1. Since F was estimated from the individual time of performance, vEND could be calculated. The average speed of performance (vMIG) and vEND (m . s-1) were found to be linearly correlated: vMIG = 1.12 + 0.64 vEND (r2 = 0.72; n = 36). The variability of vMIG explained by vEND, as measured by r2, is greater than that calculated from any one regression between vMIG and VO2max (r2 = 0.51), F . VO2max (r2 = 0.58), or VO2max . C-1 (r2 = 0.63). The mean ratio of observed (vMIG) to theoretical (vEND) speeds amounted to 0.947 +/- 0.076 and increased to 0.978 +/- 0.079 (+/- S.D.; n = 36) when the effects of air resistance were taken into account. It is concluded that vEND = F . VO2max . C-1 is a satisfactory quantitative description of the energetics of endurance running.