ABSTRACT The histochemical fluorescence method of Hillarp and coworkers for the demonstration of catecholamines (CA) and 5-hydroxytryptamine (5-HT) has been used together with histochemical criteria and with a pharmacological approach to the problem. It has been possible to show that primary CA are probably stored in the PAS-positive cells of the adenohypophysis of some mammals. In the cat, the great majority of the PAS-positive cells have this property. The amines are not depleted by treatment with reserpine, metatyrosine or α-methyl-meta-tyrosine. The storage mechanism for CA in the PAS-positive cells is discussed. The PAS-positive cells of the anterior lobe of the mouse also have the property of forming CA from L-3,4-dihydroxyphenylalanine (L-DOPA) but not from D-DOPA. Some of these cells are also able to decarboxylate 5-hydroxytryptophane (5-HTP) to 5-HT.