In a previous study (Mariano et al, 2004) we suggested that mitochondria isolated from embryogenic cell cultures of Araucaria angustifolia contain a plant uncoupling protein similar to that described by Vercesi et al, 1995. These data were obtained via oxygen consumption experiments using a Clark-type electrode connected to a Gilson oxygraph, and by membrane potential determination with safranine O using a Shimadzu RF-5301 PC spectrofluorophotometer. This protein (PUMP) was now identified with a polyclonal antibody of Arabidopsis thaliana (AtPUMP) using Western blot analysis. It was visualized as a band corresponding to a molecular mass of  approximately 32 kDa. This finding directs this ancient gymnosperm into the group of plants containing PUMP. Also studied were NAD(P)H dehydrogenases, insensitive to rotenone, in mitochondria isolated from A. angustifolia. The effect of flavone (2-Phenylchromone) on FCCP-uncoupled respiration and with 2 mM NADH as substrate was determined in the presence or absence of rotenone. In the presence of 500 mM flavone plus rotenone respiration was inhibited by ~60%. Subsequent addition of 10 mM succinate restored the oxidative capacity of the respiratory chain. Respiration due to succinate oxidation was then partially inhibited by 2 mM cyanide and totally inhibited on the addition of 2 mM SHAM, indicating the presence of alternative oxidase (AOX). In the absence of rotenone the flavone had no effect. This might suggest that the activity of Complex I affects the inhibition of the external NADH dehydrogenase by the flavone, but needs further investigation. Similarly, in the absence of the protonophore FCCP mitochondria were not sensitive to 500 mM flavone. Our results thus indicate that mitochondria have to be uncoupled for flavone to exert its inhibitory effect.