After 5-hydroxy-L-tryptophan (5-HTP) and L-3,4-dihydroxyphenylalanine (L-DOPA) were injected i.p. in the laboratory shrew Suncus murinus, immunocytochemical and immunofluorescence studies were conducted on continuous or same sections of the brain, using specific anti-tyrosine hydroxylase (TH), anti-aromatic L-amino acid decarboxylase (AADC), anti-dopamine (DA) and anti-serotonin (5-HT) antisera which were produced in our laboratory. The results of double-staining by the immunofluorescence method as well as immunoelectron microscopy strongly indicate that the cells of the premammillary nucleus of the laboratory shrew brain (AADC-only-positive neurons) are capable of synthesizing DA and 5-HT simultaneously upon simultaneous administration of L-dopa and 5-HTP.