Corn gluten meal (CGM) has been shown to be an effective natural preemergence herbicide and fertilizer for various plant production systems. Alaninyl-alanine (Ala-Ala), along with four other dipeptides, were isolated from CGM and identified as being the inhibitory compounds. The herbicidal effects are seen as growth-regulating, root inhibitors that have minimal effect on shoots at low concentrations. Little is known about the inhibitory action of CGM or Ala-Ala;The objective of the first phase of this research was to elucidate morphological and anatomical differences in perennial ryegrass seedlings treated with Ala-Ala using light and transmission electron microscopy, as well autoradiographic studies using (3H+) -Ala-Ala. Results from these experiments described the treated root tips as being void of cellular components, specifically discernible nuclei and mitotic structures, with an overall loss of cytoplasmic integrity. Furthermore, root tips had extreme cell wall abnormalities including uneven thickening and breakage. Autoradiographs suggested that at high treatment concentrations causing epidermal tissue damage, there was minimal inward movement of dipeptide. At lower concentrations, root tip epidermal necrosis was not evident, and inward movement of Ala-Ala was not impeded;The objectives of the second phase of research were to use time-course studies to monitor the mitotic activity of roots treated with Ala-Ala; to use light and transmission electron microscopy to describe Ala-Ala induced changes in root cell ultrastructure; and to make comparisons with other reported modes of action of synthetic herbicides. Results showed that Ala-Ala exhibited activity on mitosis within 4 h of exposure, and by 6 h, reduction in the number of mitotic figures was nearly 100%, resulting in only interphase cells. Microscopic analysis revealed profound treatment effects. By 12 h, dense droplets, presumably membrane lipids, were visible in vacuoles and intercellular spaces. After a 48 h exposure, epidermal and cortical cell elongation in treated roots appeared to occur perpendicular to the normal elongation plane, possibly resulting from a loss of cell polarity. Root lateral branching was also noted after a 48 h exposure time.