Unrolling of 1 cm sections, taken between 3 and 4 cm from the apex, of 6‐day‐old, etiolated barley leaves, was promoted by blue (426 nm) and red (658 nm) light. Accompanying such unrolling was a reduction in the level of the free proline of the tissue. When leaf unrolling was prevented by irradiation with far‐red (728 nm) light, or treatment with abscisic acid (ABA) following red light irradiation, the level of proline remained more or less unchanged, at the level of the untreated, dark controls. The proline analogue, azetidine carboxylic acid (AZC) powerfully inhibited the light induced leaf opening, emphasizing the significance of proline‐containing, structural and functional proteins in barley leaf unrolling. The inhibition imposed by AZC is partially reversible by added