Total cobalamin and methylcobalamin levels were determined in tissues of male F344 rats fed a complete, amino acid-defined diet or a diet deficient in methionine, choline and/or cyanocobalamin. Total cobalamin levels in rats fed the complete diet were (picograms/milligram tissue +/- SEM): liver, 67 +/- 13; kidneys, 738 +/- 133; spleen, 23 +/- 2; and adrenals, 268 +/- 36. Corresponding methylcobalamin levels were: liver, 1.6 +/- 0.5; kidneys, 107.6 +/- 22.2; spleen, 0.3 +/- 0.1; and adrenals, 26.9 +/- 5.3; these values represent 2.4, 14.5, 1.4 and 9.7%, respectively, of the total cobalamin levels. Total cobalamin levels of all tissues studied were altered by cobalamin deprivation alone or in conjunction with methionine and/or choline deprivation. Methylcobalamin levels were more resistant to dietary alteration. Regardless of the presence or absence of methionine and cobalamin in the diet, choline deprivation always decreased the proportion of methylcobalamin in the liver. Kidney levels of methylcobalamin, like those of total cobalamin, were decreased by removal of cobalamin from the complete or the methyl-deficient diets. The results demonstrate that cobalamin, methionine and choline exert quite different effects on tissue levels of the cobalamins in rats.