Chlorophyll (Chl) fluorescence from greening and fiilly green barley leaves was analyzed using a phase-modulated fluorimeter. Inunediately following the irradiation of fiilly-etiolated leaves, a pool of (newly-synthesised) Chl/chlorophyllide was detected with a fluorescence lífetime similar to that of these pigments in solution. After some hours, the major fluorescence component was a short-lifetime species which we believe to represent photosystem 2. Selective inhibition of polypeptide synthesis in either the chloroplast or cytosol greatly reduced the extent of Chl assimilation and gave rise to novel fluorescence-lifetime profiles which are believed to arise flom complexes in various stages of assembly. Hence angiosperms háve evolved control systems to lim't the appearance of free Chl; this involves close co- ordination between pigment and polypeptide synthesis.