As an indirect way to establish whether catecholamines are actually elaborated in the islets of Langerhans of adult albino and pigmented mice and guinea pigs, or albino rats, the presence of tyrosine-hydroxylase was immunohisto-chemically investigated using a highly specific polyclonal serum against the enzyme. The antiserum was applied to paraffin sections of pancreases fixed either in acrolein or Bouin’s fluid. Such sections were then treated with the avidin-biotin method. Tyrosine-hydroxylase was found exclusively in the β cells of rats and in those of the two strains of mice. Results in the guinea pig varied according to whether the animals were pigmented or albino. In pigmented specimens, the enzyme was detected in β and non-β cells, whereas in albino animals it was undetectable in any of the islet cells. All these observations were performed in material fixed in acrolein. Bouin’s fluid resulted a rather poor fixative for the detection of the enzyme in the islets. The results are compared with those of other authors who investigated the presence of monoamines in the islets of Langerhans. Since tyrosine-hydroxylase is a specific marker for catecholamine-synthesizing cells, it is concluded that insular cells have the ability to elaborate these substances.