Tryptophan and serotonin were imaged with infrared illumination by thr
ee-photon excitation (3PE) of their native ultraviolet (UV) fluorescen
ce. This technique, established by 3PE cross section measurements of t
ryptophan and the monoamines serotonin and dopamine, circumvents the l
imitations imposed by photodamage, scattering, and indiscriminate back
ground encountered in other UV microscopies. Three-dimensionally resol
ved images are presented along with measurements of the serotonin conc
entration (similar to 50 mM) and content (up to similar to 5 X 10(8) m
olecules) of individual secretory granutes.