1. In vitro leucine catabolism in adipose tissue, skeletal muscle, kidney and liver homogenates was studied in sheep.2. In Expt 1, Suffolk × Targhee ram lambs were slaughtered at 1, 28, 56, 84, 112, 140, 168, 196, 224 and 365 d of age. In Expt 2, 5-month-old crossbred wethers were fed on 80, 120 or 180 g crude protein (nitrogen × 6·25) /kg diets for 4 weeks or fed on 120 g crude protein/kg for 4 weeks and then fasted for 48 or 96 h before slaughter. Leucine catabolism was assayed in tissue homogenates for Expts 1 and 2.3. Leucine deamination (per unit protein) was highest in skeletal muscle at day 1 and then declined; liver exhibited an activity pattern akin to muscle while kidney activity tended to rise over the duration of the study. Adipose tissue in vitro leucine deamination was higher at all ages studied and 9- to 50-fold higher than all other tissues at 365 d. Leucine decarboxylation (per unit protein) was highest at day 1 in muscle and declined to low levels (P< 0·01) after 28 d; liver and kidney decarboxylation activities were higher than muscle at all ages with kidney showing the highest activity. Whilst adipose had high initial activity it declined significantly (P< 0·01) by day 28 and remained low.4. Dietary protein intake had no effect on leucine deamination in any tissue. Leucine decarboxylation tended to increase with protein intake for all tissues except kidney. Length of fast (96 h) resulted in a variable decline in leucine deamination; leucine decarboxylation was significantly lower in kidney, liver and adipose tissue after a 96 h fast.5. When these in vitro enzyme activity results are related to questions concerning the role of skeletal muscle in leucine catabolism in sheep, the present findings indicate that in grown sheep, skeletal muscle has a small to moderate role in total body leucine deamination but plays a very minor role in leucine decarboxylation. Adipose tissue appears to be a major site of leucine deamination in grown sheep. These results are not in agreement with the idea that during fasting leucine becomes an important energy substrate and is oxidized in skeletal muscle as has been shown in rodents.