The rate of oxidation of the carboxyl and methyl carbons of [14C]methionine to CO2 by homogenates of liver, kidney cortex, pancreas, muscle and small intestinal mucosa was studied in two breeds of sheep (Merino and Poll Dorset Horn) at three ages (2 weeks, 3 months, 4 years). Sodium alpha-keto-gamma- methiolbutyrate (0 X 4 mM) stimulated production of CO2 from the carboxyl carbon of methionine, but not from the methyl carbon. Sodium pyruvate did not affect the recovery of CO2 from either carboxyl or methyl of methionine. Sodium formate (15 mM) suppressed the conversion of the methyl carbon of methionine to CO2 by liver and kidney homogenates to 4 and 50%, respectively, of control values, but did not affect the percentage of carboxyl carbon of methionine recovered in CO2 with either tissue. With addition of S-methyl-L-cysteine (40 mM) and 3- methylthiopropionate (10 mM) the percentage of methyl and carboxyl carbons recovered in CO2 was reduced to about 20% of control values in homogenates of both tissues. Activity per gram of tissue was higher in liver and kidney cortex than in pancreas, intestinal mucosa, or muscle, with no significant differences due to breed (Merino or Poll Dorset Horn) or sex (ewe, ram or wether) of sheep. Conversion of both the carboxyl and methyl carbons to CO2 by liver was significantly lower in 2-week-old lambs than in older animals (P less than 0.01). The activity of other tissues was not markedly affected by age. Results are discussed in relation to evidence of alternative pathways of methionine catabolism, and capacities of the tissues of the sheep to catabolize methionine by alternative pathways.