Plasma, brain and liver microsomal phospholipid content has been studied in rats fed a choline-deficient, choline-supplemented or ordinary laboratory diet for about half a year, using lipid extraction, thin-layer chromatography and phosphorus determination. Microsomal protein and cytochrome P450 content as well as NADPH cytochrome C reductase (EC 1.6.2.3) and ethylmorphine-N demethylase activity have been determined additionally. 35 S-thiopentone (5-ethyl-5-[1-methylbutyl]-2-thiobarbituric acid monosodium salt) half-life has been calculated following up either 35 S-thiopentone plasma level or fractional urinary excretion of 35 S-thiopentone metabolites. Choline deficiency leads to decreased plasma and brain phospholipid content. In the liver only a decreased phospholipid concentration was obtained. Total microsomal phospholipid content in the liver remained unchanged. Compared with the animals fed on a common laboratory chow both choline-deficient and choline-supplemented diets lead to an increase in total microsomal protein. Total cytochrome P450 content in the liver showed normal values in the choline-deficient group, an increased total amount of NADPH cytochrome C reductase, and TV-demethylation of ethylmorphine in a normal range. Feeding the choline-substituted diet was followed by a significant decrease in microsomal P450 content. Ethyl-morphine-N-demethylase was diminished to about 35 %. In vivo 35 S-thiopentone half-life was normal (approx. 15 h) in the choline-deficient group but distinctly prolonged in the choline-supplemented group (approx. 23 h).