Corn (Zea mays L.) seedlings possessed a high degree of tolerance to growth inhibitory effects of pre-emergent application of 2-chloro-N-isopropylacetanilide (propachlor), squash (Cucurbita maxima Duchesne) was intermediate in its response to the herbicide followed by oats (Avena sativa L.), lambsquarters (Chenopodium album L.) and redroot (Amaranthus retroflexus L.). Root growth was more severely retarded by lower concentrations of propachlor than was stem development. Propachlor inhibited cell division in onion (Allium cepa L.) root tips. Aberrations in the mitotic behavior, such as contraction of chromosomes, was also induced. Cell elongation, in particular auxin induced cell elongation of oat coleoptile , was inhibited in direct proportion to propachlor concentration. Concomitant with the inhibition of squash seedling growth and development, propachlor prevented the normal senescence of squash cotyledons. Cell wall expansion and breakdown and the utilization of proteinaceous and lipid reserves in the cotyledons were inhibited by propachlor treatment. Amino acid activation, both in vivo as well as in vitro, was significantly reduced in squash whereas in corn, amino acid activation was inhibited only in vitro. This inhibition of amino acid activation is postulated to be the primary mechanism of propachlor's herbictdal action. This inhibition would prevent the formation of proteinaceous compounds including (1) the enzyme complexes which would break down storage material , and (2) alter cell walls to allow cell elongation.