The influence ofadenosine diphosphate (ADP) on respiration of pancreatic acinar cell mitochondria in situ was studied. The model of digitonin-treated pancreatic acini was used. It was found that succinate or a mixture of pyruvate, glutamate and malate intensified respiration ofpermeabilized cells. Low ADP concentration (100 microM) did not influence the rate of oxygen uptake, whereas at higher concentration (750 microM) brief intensification of respiration was observed when using nominally Ca(2+)-free medium. When the medium with 100 nM Ca2+ was used, ADP had no effect on oxygen uptake, while the rate of respiration stimulated by a mixture of pyruvate, glutamate and malate increased. Rate of succinate-stimulated respiration did not depend on Ca2+ content in medium. The presence of ATP in the medium reduced the stimulatory effect of ADP, but increased its duration. Intensification of respiration by ADP, occurred only at elevated Ca2+ content, was not associated with oxidative phosphorylation because oligomycin did not inhibit it. The effect ofADP might be a novel "functional marker" of development of pathological processes in the mitochondria of acinar pancreacytes.