A non-green catabolite of chlorophyll (Chl) the fluorescent compound FC 2, is produced when intact senescent chloroplasts of barley (Hordeum vulgare L.) are incubated in the presence of ATP. The origin of FC 2 has now been demonstrated by employing senescent chloroplasts containing Chl (14)C-labelled in the pyrrole-rings: upon incubation in the presence of ATP, (14)C-labelled FC 2 is generated. The production of FC 2 requires the hydrolysis of ATP as demonstrated by the failure of the β, γ-imido analogue to support the reaction. Adenosine triphosphate can partially be replaced by UTP but GTP and CTP, as well as ADP and AMP, are ineffective. The system responsible for FC 2 production can also be fueled with glucose-6-phosphate, fructose-6-phosphate and glucose-1-phosphate; other sugar-phosphates including glyceraldehyde-3-phosphate have no effect. Adenosine triphosphate is also required for the release of FC 2 from chloroplasts. When chloroplasts are incubated in the presence of UTP or hexose-monophosphates which support the generation of FC 2 within the organelles, the catabolite is not released. It is concluded that the envelope of senescent chloroplasts is equipped with translocators for the cytosolic compounds which provide the metabolic energy and cofactors required for the action of the catalyst(s) responsible for the oxidative cleavage of Chl-porphyrin and possibly also for the dismantling of Chl-protein complexes. Moreover, a translocator may be involved in the release of the primary catabolites of Chl from chloroplasts.