Quantitative microfluorimetric studies were carried out on the formaldehyde induced fluorescence of dopamine in nerve terminals of the nuc. caudatus putamen using the technique of Falck and Hillarp. After tyrosine hydroxylase inhibition produced by alpha-methyl-p-tyrosine (H 44/68) a time-dependent disappearance of the dopamine fluorescence occurred in an exponential manner, T1/22.6 hr. Apomorphine treatment resulted in a considerable counteraction of the H 44/68 induced reduction of the fluorescence, whereas treatment with haloperidol potentiated it. Administration of gamma-hydroxybutyrolactone led to a marked increase of the dopamine fluorescene. The present microfluorimetric results were in perfect agreement with chemical-analytical determinations of dopamine carried out under identical experimental conditions, and with those reported previously. The fluorescence intensities obtained in the nuc. caudatus putamen were found to be in the linear part of the dopamine fluorescence concentration relationship as observed in protein models. It may be concluded that by using microfluorimetric quantitation of the formaldehyde induced fluorescence in the nuc. caudatus putamen it is possible to obtain a reliable quantitation of the relative amount of dopamine in the dopamine nerve terminals.