The effects of decursinol and decursin on chloroplast-mediated electron transport and phosphorylation in barley seedlings were investigated in comparison with coumarin in the dark or light. The changes of CP-complexes were also studied. Decursinol, decursin and coumarin caused marked inhibitory effects on germination of seed and electron transport and phosphorylation activity of seedling. The following order of inhibitory effectiveness was exhibited; decursinol>coumarin>decursin. Loss of chlorophyll and decrease of electron transport activity were retarded in the dark, but were reversely accelerated in the light by these three chemicals. The changes of CP-complex patterns were also similar to effects on chlorophyll content and the electron transport activity. These opposite effects in the dark and light suggest that these three chemicals act as natural growth retardants rather than cytokinins or growth inhibitors.