Lipid fluidity in rat liver microsomes was assessed by the steady state fluorescence polarization of stearic acid labelled at positions 2, 7 or 12 with 9-anthroyl groups. Oxygen solubility and/or mobility was determined by measuring the quenching rate of different pyrene derivatives containing an aromatic group separated from the microsome surface by 1, 4 or 11 methylene groups. The fluorescence of these compounds, as well as that from methylpyrene and benzo(alpha)pyrene, was quenched by carbon tetrachloride. The effect of the latter compound and of n-heptanol on the fluorescence polarization of the anthracene derivatives and the quenching rate by oxygen was also determined. The results obtained indicate that the lipid order decreases and the solubility and/or mobility of oxygen increases towards the bilayer core. The addition of carbon tetrachloride decreases the order of the membrane and increases the rate of fluorescence quenching by oxygen. The largest effect of the additive on the fluorescence quenching rate by oxygen is observed at intermediate positions in the bilayer. n-Heptanol addition decreases the membrane order and increases the rate of fluorescence quenching by oxygen or carbon tetrachloride, the maximum effect being observed at the microsomal surface. The differences between the effects of the two additives are discussed in terms of their different localizations. Fluorescence quenching by oxygen is considerably more affected by carbon tetrachloride than fluorescence depolarization. In addition, the maximum effect takes place at different positions in the membrane.(ABSTRACT TRUNCATED AT 250 WORDS)