Mitochondrial respiration with succinate as substrate increased and the mitochondrial protein content declined in the brain of the ageing male garden lizard. The lactate/pyruvate ratio of both blood and brain did not show appreciable changes with advancing age. Cold-shock treatment induced a significant rise in the respiratory rate of brain mitochondria of young but not appreciably in that of middle-aged and old lizards. On the other hand both mitochondrial protein content and lactate/pyruvate ratio increased only in the brain of cold-shocked lizards of the old age-group. The lactate/pyruvate ratio in blood remained unaltered following cold-shock treatment of lizards of all three age-groups.