1. A modification of an aqueous isolation procedure of chloroplasts is described. It results in chloroplast preparations of high purity and good maintenance of structure. 2. The activity of NADP-GPD is increased with the same kinetics by irradiance of isolated spinach chloroplasts as by irradiance of the intact organism. 3. Irradiance causes a decrease in NAD-GPD-activity in the isolated chloroplasts. The change in the turn-over rate (Δ A1 of NADP-GPD-increase: Δ A2 of NAD-GPD-decrease) is 5:1. This ratio is of the same order of magnitude as that obtained earlier by irradiance of whole Lemna gibba (Δ A1 : Δ A2 = 3,5 : 1). Evidence is given that the NAD-GPD in the cytoplasm is not influenced by light.