The ligand-dependent dissociation of spinach chloroplast glyceraldehyde-phosphate dehydrogenase (Mr 600000) to protomers of Mr about 145000, previously shown by us in 1973, has been further characterized by the technique of velocity sedimentation in sucrose gradients. The process exhibits cooperativity and is accompanied by an increase of the apparent NADP+-dependent activity (reactivation) from a ratio of 0.1-0.2 to a ratio of 1 to 2 with respect to the NAD+-dependent activity. In addition to NADP+ and NADPH, most nucleotide triphosphates and, to some extent, Pi, act as dissociating agents. The enzyme is depolymerized and progressively inactivated in the presence of 2'-AMP. 2. Incubation with 20 mM dithiothreitol or 8-10 mM GTP increases the apparent NADP/H)-dependent activity, although addition of a small amount of a dissociating compound, such as 0.06 mM NADP+, is required for depolymerization. 3. NAD+, NADH and, to a lesser extent, glyceraldehyde 3-phosphate, NMN and cyclic AMP act as inhibitors of the dissociation and reactivation, however induced. They also favour the reassociation of protomers to tetramers. 4. The NADP(H)-linked activity is probably a property of the protomers only. The system described here resembles in many respects the light-dependent regulation of the NADP(H)-linked activity in vivo.