Root application of N,N-dimethyl-2,2-diphenylacetamide (diphenamid) caused reduction of root and shoot growth of oats (Avena sativa L., var. Jaycee) seedlings. Shoot application did not affect plant growth, but studies with labeled diphenamid showed that diphenamid will enter also through the shoot. In ivyleaf morningglory (Ipomoea hederacea L.), a considerable amount of label was translocated from the roots to the shoots, but not in oats seedlings. The difference in tolerance between these two species (oats susceptible, morningglory resistant) may lie in the ability of morningglory to translocate diphenamid out of the roots into the shoots faster than oats. The inhibitory effect of diphenamid was restricted to the site of uptake. Reduction in shoot growth of treated plants was the result of the limited root system and it was not a direct effect of diphenamid. Diphenamid was 10 times as toxic to oats as its metabolites. Oats seedlings inhibited by diphenamid for up to 5 days, and then placed in water recovered from the diphenamid caused inhibition. The resumed root growth appeared to be normal. The amount of uptake of 14C-labeled sucrose by excised roots treated with 10−5 M diphenamid was equal to that in untreated roots; however, more sugar was incorporated into the untreated roots than the treated roots. It appears that diphenamid is a reversible metabolic inhibitor; it inhibits cell division in the root tip perhaps by limiting utilization of substrates in the cells.