SummaryWeakness, easy fatiguing, and lack of endurance are commonly perceived by patients with Parkinson's disease (PD). Although the slowed motor repertoire in PD may underlie these experiences, other abnormalities in skeletal muscle utilization also may be involved. We investigated whether an index of metabolic efficiency during a continuous exercise task, the latency until anaerobic threshold (AT), is altered by L-DOPA (LD). While pedalling a bicycle ergometer against a uniform workload, subjects were monitored for expired O2 and CO2. As compared to an unmedicated state, LD treatment delayed AT by a mean (±SE) of 5.67 ± 0.89 to 6.62 ± 1.23 min (p < 0.05, paired t test). Subjects did not differ in their perceived exertion upon reaching AT. With relief of parkinsonism by LD, the efficiency of energy utilization is also increased in exercised skeletal muscle.