The in vitro effects of piperine on three bioenergetic reactions namely, oxidative phosphorylation, ATPase activity and calcium transport by isolated rat liver mitochondria have been investigated. Piperine was found to inhibit state 3 and DNP-stimulated respiration by mitochondria respiring with glutamate plus malate or succinate as substrates. The I50 values of piperine on oxidative phosphorylation in the presence of glutamate plus malate and succinate were 22 and 12 micrograms/mg mitochondrial protein respectively. With HTM preparations, the oxidation of added NADH and succinate was depressed by piperine while ascorbate plus TMPD oxidation was slightly affected. Piperine did not inhibit the mitochondrial ATPase activity induced by DNP, but by itself exerted stimulating activity on this enzyme. Piperine was also found to diminish calcium uptake and to facilitate the release of accumulated calcium by the mitochondria incubated with succinate or ATP. These results suggest that piperine inhibits mitochondrial oxidative phosphorylation at the level of respiratory chain, and the inhibitory site(s) is in the segment(s) ahead of cytochrome C. The mechanism of the piperine-induced ATPase activity is not known; but the effect of piperine on calcium transport is likely to be consequential to the effects of this compound on the mitochondrial respiratory chain and ATPase activity.