Determining the basis of intraspecific herbicide tolerance was expected to be a useful way of revealing factors which are regulated by safeners in providing their protective effect. Differential tolerance to the chloroacetanilide herbicide metolachlor and the thiocarbamate herbicide EPTC was examined in 11 corn hybrids. Tolerance to one of these herbicides does not imply similar tolerance to the other. Detoxication of these herbicides in plants is mediated via conjugation with glutathione (GSH). GSH levels from 1.8 to 2.4 µmol/g fresh weight were determined for the eleven corn hybrids tested. There was no correlation between GSH content and herbicide tolerance. The monooxygenase inhibitor piperonyl butoxide (PBO) acted synergistically with EPTC on 8 of the tested corn hybrids. A Similar antagonism by the oxygen evolving compound calcium peroxide provided additional evidence for the importance of oxidative processes in EPTC tolerance which were not important in determining metolachlor tolerance. The more rapid absorption and greater accumulation of ¹⁴C-metolachlor by 'Northrup-King 9283' corn relative to 'Cargill 7567' corn at least partially explains the increased susceptibility of the former hybrid to metolachlor. The in vitro metabolism of ¹⁴C-metolachlor was similar for both hybrids. A lag in the expression of glutathione S-transferase (GST) activity during early seedling development of 'Northrup-King 9283' corn may be of additional significance in its limited tolerance to metolachlor. The safener benoxacor was effective in protecting 'Northrup-King 9283' and other susceptible corn hybrids from metolachlor injury. Benoxacor had no effect on metolachlor uptake or the rate of non-enzymatic conjugation of metolachlor. Seedlings of 'Cargill 7567' and 'Northrup-King 9283' treated with 1 µM benoxacor metabolized metolachlor to the GS-conjugate at a rate 1.7 times that of untreated seedlings. GST activity was stimulated by 35% by similar treatment. GST isozymes with metolachlor conjugating activity (GST-metolachlor activity) were found in the cytosol and microsomal fractions of corn extracts. At least two GST-metolachlor isozymes were separated by DEAE-Sepharose chromatography. The activity of both isozymes was increased by benoxacor treatment. It appears that benoxacor regulates metolachlor tolerance by inducing GST isozymes that consequently increase the rate of metolachlor detoxication. / Ph. D.