The aim of this study was to test the hypothesis that the self selected speed in running (vss) is dependent upon the same factors that determine maximal speed in endurance events (e. g. the anaerobic threshold). Experiments were carried out on 8 recreational long distance runners (42.1 +/- 8.6 years of age, 70.1 +/- 10.6 kg of body mass, 1.74 +/- 0.06 m of body height) while they were participating in a 14 day relay race. During the "race" the subjects were not requested to perform maximally but only to cover their running turn (1 hour per day) at their preferred pace. The relationships between heart rate (HR), perceived exertion (RPE), blood lactate concentration ([La]b) and speed (v) were determined in each subject, before the race, during an incremental running test. From these relationships the speed corresponding to a 4 mM concentration of lactate in blood (v4mM) was calculated and found to be 14.3 +/- 1.8 km x h(-1) (n = 8). At this speed the RPE and HR values were 13.6 +/- 1.4 and 156.4 +/- 12.8 bpm, respectively. The average values of speed (vss, 13.4 +/- 0.6 km x h(-1)), RPE (13.5 +/- 1.4) and HR (154.4 +/- 7.6 bpm) measured during the race (n = 47) were not significantly different from those measured at the lactate threshold (v4mM, RPE4mM and v4mM). However, vss and the average HR during the race showed significantly lower variances than v4mM and HR4mM suggesting that, besides the need of avoiding lactate accumulation in blood, other factors must be involved in the choice of speed in running.