Protoplasts, isolated from the primary leaves of darkgrown wheat, have been used as a model system to study signal transduction of phytochrome. Research has been focused on changes in plasma membrane properties and the role of Ca2+. After exposure to red light (R), protoplasts swelled only when Ca2' was present in the medium. Far-red light (FR) inhibited swelling after exposure to R. R-induced swelling was also inhibited by the Ca2+-channel blockers Verapamil and nifedipine. Swelling was induced in darkness by the Ca2+-channelagonist Bay K-8644. A GTP-binding protein is thought to be also involved as GDP-p-S, known as an inhibitor of G-proteins, inhibited the R-induced swelling, and the activator GTP-y-S stimulated swelling in darkness. Li+, neomycin and H„ known as as antagonists of the phosphatidyl inositol cycle in animal cells, inhibited the R-induced swelling. PMAL, an agonist, induced swelling in darkness. Using the Ca! '-sensitive dye murexide, an increase in [Ca2 * ]m„lluI„ was found after R-irradiation, which was reversed by FR, This effect was inhibited for *80% by the Ca2+-channel blocker Verapamil and the calmodulin antagonist W7. After R-irradiation an increase in [Ca2+]cjl was found in oat protoplasts as measured with the Ca2+-indicator quin2 (Chae el al. 1990, Biochim. Biophys. Acta 1051: 115-122). FR reversed this effect.