The inhibiting activity of eight derivatives of 2,2,4-trimethyl-1,2-dihydro- and 1,2,3,4-tetrahydroquinolines with alkoxy and hydroxy substituents in the 8- or 6-position on the aromatic ring was compared in the auto-oxidation of n-decane at 150degreesC and P-carotene at 50degreesC and initiated oxidation of ethylbenzene at 60degreesC. It was found that di- and tetrahydroquinolines hydroxylated in the 6- or 8-position and alkoxy-substituted in the 6-position exhibit high antiradical activity. 8-Methoxyhydroquinolines have low antiradical and inhibiting activity. Tetrahydroquinolines are more effective inhibitors than dihydroquinolines and display activity over a wider temperature range. Hydroxylated hydroquinolines interact with radicals via the hydroxy group, and the heterocycle acts as an electron-donating substituent activating the O-H bond. The high antiradical activity of 6-ethoxylated hydroquinolines is accounted for by the conjugation of the lone electron pairs of N and O atoms in the para-position of the aromatic cycle and by the electronic-structure stabilization of the aminyl radical formed in radical reactions, as in the case of tocopherols and 6-hydroxychromans.