(A) In vivo P/O ratio decreases 24 hours after PQ-treatment in young, middle aged, and old mice. Reduced P/O is associated with reduced resting ATPase rate (B) without significant changes in mitochondrial O2 consumption (C) following PQ treatment. The same trend led to a significant decrease in P/O of old relative to young control mice. Data presented as mean ± SEM, n = 4-8, * p< 0.05, **p<0.01 relative to age-matched control, #p<0.05 relative to young control.