The plasticity of phenolic acid concentration in plants under various physiological stress situations raises questions concerning the ecological significance of such behavior. Helianthus annuus L. is remarkably plastic with regard to chlorogenic and ischlorogenic acid concentrations when subjected to nitrogen deficiency, NaCl-stress, short exposures to UV radiation or to combinations of these stresses. Stress due to NaCl has not previously been reported to produce this effect.I propose that chlorogenic acid is representative of a group of chemicals which have originated as regulators of various metabolic systems under stress and have subsequently acquired allelochemic properties against pathogens, herbivores, and competitors. If this hypothesis is correct then specific patterns of distribution with regard to concentrations of phenolic acids should be found. Some such predictions are discussed and means of distinguishing between various selective agents are explored. Generally, if natural environmental stress stimulates phenolic acid concentrations and hence more efficient growth patterns, then for several kinds of variables, higher concentrations should be found in more rather than less stressful habitats. If biological agents are significant agents of natural selection for this group of substances, such a pattern should not occur in some cases.