Freshly isolated mitochondria from guinea pig heart were resuspended and analyzed as described in Methods and the legend of Fig. 3. A) Under FET (G/M) and RET (Succ), H2O2 and several other bioenergetic variables (e.g. ΔΨm, NAD(P)H) were monitored simultaneously with a spectrofluorometer (see legend Fig. 3 and Methods). B) In parallel, and under similar conditions, O2 measurements were performed. Mitochondrial respiration was measured in state 3 (St3) and state 4 (St4). Shown are the results obtained with H2O2 (A) and O2 (B) specific fluxes. The numbers in grey represent the H2O2 detected as a percentage of the O2 consumed (see Methods). The number of samples, n, analyzed was n=4 (2 experiments) and n=6 (2 experiments) for panels A and B, respectively. * p<0.05; ** p<0.01; *** p<0.001.