Cucumber (Cucumus sativus L. ‘Chicago pickling’) and wheat (Triticum aestivum L. ‘Manitou’) seeds were germinated in various concentrations of (2,4-dichlorophenoxy) acetic acid (2,4-D), (2,4,5-trichlorophenoxy)acetic acid (2,4,5-T), 3,6-dichloro-o-anisic acid (dicamba), and 4-amino-3,5,6-trichloropicolinic acid (picloram). The nucleic acid and protein levels in the roots of the two species were determined 4 days after germination and compared on a per root basis. All four herbicides increased the DNA and protein of both cucumber and wheat roots with the increase in cucumber being higher than in wheat. The greatest difference between the tolerant wheat and susceptible cucumber was the effect on the RNA levels. As the concentration of all four herbicides increased, a progressive decrease of RNA levels occurred in wheat. In contrast, in cucumber the herbicides at high concentrations (10 and 100 ppmw) increased the RNA levels by more than 200% over the water controls. When the protein levels in both species were compared on a per unit RNA basis, there was an inverse relationship. In wheat the protein/RNA ratio was higher than the control whereas in cucumber the ratio was lower. A differential alteration of RNA species and interference with protein synthesis is suggested as the basis for selectivity.