Abstract—–Changes in glycogen, glucose, lactate and monoamine levels in various brain structures were studied after unilateral diencephalic lesions in the rat. These lesions were placed to transect the fibres of the medial forebrain bundle within the lateral hypothalamus as well as to encroach upon the medial aspect of the internal capsule. Such lesions are known to produce unilateral reduction of telencephalic monoamines rostral to the lesion. In paraformaldehyde‐perfused brain, the level of glycogen increased by 30 per cent (+0 82 μmol/g) in the caudate nucleus ipsilateral to the lesion in comparison to that in the caudate of the non‐lesioned side. Other telencephalic areas, as well as cerebellum and brainstem, showed no significant change in glycogen levels. In brains from unanaesthetized rats, frozen 15–30 s after decapitation (to simulate anoxia, with consequent glycogen mobilization), a caudate‐containing sample from the lesion side had 31 per cent (+0 51 μmol/g) more glycogen than a comparable sample from the control side. Glucose levels were also elevated by 72 per cent (+0 05 μmol/g, whereas lactate levels were not significantly changed. Similar samples obtained from animals 3anaesthetized with phenobarbital for 2 h prior to killing showed a 76 per cent increase in glycogen in both lesion and control caudate nuclei when compared to those obtained from the unanaesthetized group. Glycogen levels were increased by 15 per cent (+0 41 μmol/g) and glucose by 72 per cent (+0 31 μmol/g) on the lesion side in comparison to the values obtained from the control side in the anaesthetized group. Lactate levels were not significantly affected. The dopamine levels in these caudate samples from the lesion side were reduced by 97 and 88 per cent in the unanaesthetized and anaesthetized animals, respectively. In samples of caudate allowed to glycolyse under anoxic conditions for 10 min, the amount of lactate found on the lesion side was increased 29 per cent over that found in identical samples from the non‐lesioned side. Dopamine was reduced by 78 per cent in these samples. The remaining telencephalic areas exhibited no significant side to side difference in lactate levels despite a 71 per cent decrease in their levels of norepinephrine. These findings demonstrate that glycogen metabolism in the neostriatum is affected by diencephalic lesions which result in a loss of striatal monoamines, observations suggesting that brain biogenic amines may play a role in the regulation of neuronal metabolism