The distribution of simple esterase has been studied in the nuclei and fiber tracts of the medulla oblongata. The simple esterase activity has mainly been observed in the grey matter. The white matter did not reveal enzymatic activity. In the grey matter also the cranial nerve nuclei are intensely stained as compared to intrinsic and reticular nuclei. The distributive pattern of simple esterase in the nuclei has been discussed in relation to its metabolic involvement in brain.