Ventilatory threshold, apparent mechanical efficiency, oxygen debt repayment, heart rate and perceptions of exertion at the ventilatory threshold have been examined in 8 men and 8 women during the performance of four types of exercise (2-leg, 1-leg, arm plus shoulder, and arm ergometry) under normoxic and hypoxic (12% oxygen) conditions. The ventilatory threshold (percentage of task-specific VO2peak at which a disproportionate increase of ventilation begins) was not significantly affected by the sex of the subject, by hypoxia, or by the volume of active muscle involved in the activity, but showed poor reproducibility in small muscle tasks. The apparent mechanical efficiency in 2-leg ergometry was increased from 25.7 to 28.1% under hypoxic conditions, presumably reflecting an increased contribution of anaerobic effort to sub-maximal work. However, oxygen debt repayment following exhausting exercise was much smaller for arm than for leg ergometry. The heart rate corresponding to the ventilatory threshold decreased as the volume of active muscle was reduced. General and respiratory perceptions of effort were rather light for self regulation of an exercise prescription to the ventilatory threshold, and particularly with tasks involving the arm muscles, prescription may best be regulated by the intensity of local muscular sensations.