This study aimed to validate a portable metabolic system in field measuring conditions, such as prolonged moderate exercise at low temperatures, high humidity and with external wind. VO(2), VCO(2), RER and V (E) were measured using the Oxycon Mobile (OM), with a windshield, during cycle ergometer exercise: (1) indoors at three submaximal workloads with no wind or with external wind (13-20 m s(-1)) from front, side and back; (2) at two submaximal workloads outdoors (12 ± 2°C; 86 ± 7% relative humidity (RH)), with and without a system for drying the ambient air around the air sampling tube; and (3) at one workload outdoors for 45 min (5 ± 4°C; 69 ± 16.5% RH). Any physiological drift was checked for with pre- and postmeasurements by the Douglas bag method (DBM). A minor effect of external wind from behind was noted in RER and V (E) (-2 and -3%). The system for drying the ambient air around the gas sampling tube had no effect on the measured levels. A small difference in VCO(2) drift between the OM and DBM (1.5 mL min(-2)) was noted in the stability test. The results indicated that heavy external wind applied from different directions generally does not affect the measurements of the OM and further that, when using a unit for drying the ambient air around the gas sampling tube, the OM can accurately measure VO(2), RER and V (E) at submaximal workloads for at least 45 min under challenging conditions with regard to humidity and temperature.