The biologically active cis-cinnamic acid (cis-CA) has been perceived as a synthetic plant growth regulator for decades. However, in the present study, we found that cis-CA actually exists as a naturally occurring compound in a Brassica plant. This natural growth- regulating substance presents in both the sunlight-irradiated leaf tissue and the non-irradiated root tissue. The concentrations of cis-CA in both tissues are comparable to the biologically effective levels of those major plant hormones. The presence of cis-CA in root tissue suggests that it may be produced through both light-dependent and -independent path- ways or it can be transported from a plant organ to another.