The role of mitochondria in the phosphorylation of ADP to ATP in the early steps of seed germination has been studied. Mitochondria were extracted from dry sunflower (Helianthus annuus) seeds. Adenylate kinase-dependent ATP synthesis was inhibited by p(1),p(5)-di(adenosine-5')pentaphosphate. Synthesis of ATP was observed with the different substrates: citrate, alpha-ketoglutarate, succinate, malate, pyruvate or NADH. This synthesis was activated by cytochrome c, and inhibited by cyanide, oligomycin, carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone, and carboxyatractyloside. The ATP/O values with succinate were 0.85 and 1.2 in the absence or presence, respectively, of cytochrome c. Electron micrographs showed that mitochondria of dry tissues have different structures when observed in situ or in vitro after aqueous extraction, suggesting that profound changes occurred after the contact with the aqueous medium. These results confirm previous data obtained in vivo showing that mitochondria present in dry seeds are able to synthesize ATP as soon as the seeds are rehydrated.