The role of Calcium ions (Ca2+) in the phytochrome-controlled unrolling of 6-d-old etiolated barley leaf sections was evaluated through the manipulation of light treatments, Ca2+ levels and antagonists and agonists of Ca2+ metabolism. The Ca2+-chelator ethyleneglycol-bis-(Baminoethylether) - N, N, N', N', - tetraacetic acid (EGTA) inhibited the response. Ca2+ restored responsivity when supplied within 1h of light treatment. Subsequent escape from responsivity to Ca2+ involved uncoupling of measurable phytochrome from response level. A second Red- light treatment (R) given, with Ca2+, to EGTA-treated sections following Ca2+ escape from the initial R effect induced full unrolling. Similar results were obtained with the relatively independent phytochrome- controlled nitrate reductase activity induction in the sections. It is proposed that Ca2+ escape arises from phytochrome signal attenuation. Measurable Pfr and response recoupled following Ca2+-escape for the second R, given with Ca2+. This relationship was identical to that observed with a single light treatment without EGTA, despite a 50% reduction in phytochrome levels at the time of the second R. This indicated that either the ratio of Pfr/Ptot governs response induction, or that the majority of detectable phytochrome is functionally irrelevant. It is suggested that phytochrome deactivation is reversible via phytochrome cycling. A23187-induced movement of Ca2+ also induced unrolling. This, and the R effect appeared to be separate, thus Ca2+ does not mediate the phytochrome signal via simple amplitude modulation. Treatment with 1.2-Diacylglycerol (1,2,-DG) induced unrolling. The interaction of 1,2-DG and A23187 demonstrated Ca2+ sensitivity modulation, indicating involvement of Protein Kinase C. The interaction of A23187 and phytochrome did not show this. It is suggested that the induction of unrolling centres around protein phosphorylation via separate 1,2,-DG, Ca2+-sensitive and phytochrome regulated kinases.