To identify the monoamine (s) produced in the paraventricular organ (PVO) and the nucleus infundibularis dorsalis (NID) of Xenopus laevis tadpoles, formaldehyde-induced fluorescence in these hypothalamic structures was analysed by microspectrofluorometric techniques. Reference values were obtained by recording excitation and emission spectra of fluorescence in monoamine containing protein models. The maxima of the excitation and emission spectra, both under normal conditions and after treatment with HCl vapour, indicate the presence of dopamine. Based on a number of emission spectra, the PVO and NID might also contain serotonin. With regard to the functional significance of dopamine produced in the hypothalamic nuclei, it may be concluded that the catecholamine is probably identical with the melanotropin inhibiting factor (MIF).