The excised third internodes of etiolated pea seedlings were irradiated with red light (660 nm) for 40 s changing the intensity from 0, 10, 50 and 100 μmol m −2 s −1. The tissue was ground in a buffer and the plasma membrane was purified. The phosphorylation of plasma membrane proteins with molecular masses of 15, 50, 70 and 120 kDa from adenosine 5′[γ- 32P]triphosphate ([γ- 32P]ATP) at 0 °C for 7 s increased with increasing fluence rate of red light from 10 to 50 μmol m −2 s −1. However, the phosphorylation of these proteins was less enhanced at red-light fluence rate of 100 μmol m −2 s −1. Successive irradiation of the third internode by red (100 μmol m −2 s −1) for 20 s and far-red light (730 nm; 4μmol m −2 s −1) for 30 s reversed the phosphorylation of the 15 kDa protein. The 32P phosphoryl group of the 15 kDa protein which was received from [γ- 32P]ATP at 0 °C for 7 s was removed efficiently by subsequent addition at 0 °C for 5 s of cold 10 −6 M adenosine 5′-diphosphate or guanosine 5′-diphosphate and also 10 −5 M ATP or guanosine 5′-triphosphate. The phosphorylation of the 15 kDa protein in the soluble fraction showed a similar light response to that associated in the plasma membrane.