Since the 2.4 km run time test is routinely used in military training programmes as an indicator of aerobic capacity and its possible improvement, an attempt was made to: (i) establish a regression equation of VO2max v. 2.4 km run time in a group of 20 young military volunteers; and (ii) determine whether this equation could be used to predict VO2max reliably from the 2.4 km time obtained from another group. Before and after training, VO2max was measured in all subjects using a treadmill test, and 2.4 km run time was determined in the field. Linear regression equations using the 2.4 km run time as the independent variable accounted for 76-92% of the variance in VO2max, while the standard error of the estimate varied from 2.24-2.91 ml/kg/min. In the second test group, the directly measured VO2max was 59.89 +/- 0.99 ml/kg/min, while the mean value estimated from the regression equation of the first group was 59.61 +/- 1.16 ml/kg/min (P less than 0.001). It was concluded that, in the population studied, the 2.4 km run time in the field reliably predicts VO2max measured during treadmill exercise in the laboratory.