[4-Vinyl]Chlorophyllide alpha reductase (4VCR) catalyzes the conversion of 2,4-divinyl chlorophyllide alpha (DV Chlide alpha) to 2-vinyl,4-ethyl chlorophyllide alpha (MV Chlide alpha) via an NAPDH- dependent reaction. MV Childe alpha is the immediate precursor of monovinyl chlorophyll alpha in plants. In etiolated cucumber (Cucumis sativus L.) cotyledons, 4VCR is a plastidic membrane-bound enzyme. Further research on this enzyme required the development of an assay that utilizes DV Childe alpha as an exogenous substrate. Such an assay is now described. It involves conversion of exogenous DV Chlide alpha to MV Chlide alpha at high rates by etioplast membranes of cucumber, corn (Zea mays L.), and barley (Hordum vulgare L.). 4VCR exhibits high activity between 30 and 40C and in the pH range of 6.3 to 7.0. Activity is quasilinear for the first 60 s of incubation.