After administration of monoamine precursors, developing taste buds of newborn and young mice were observed by means of electron microscopy and fluorescence histochemistry. Gustatory (type III) cells occurred in the primitive taste buds during stage 1 (0-1 day after birth). These cells had an immature type of afferent synaptic contacts with nerve terminals; however, no specific fluorescence was found in the taste buds after administration of 5-HTP or L-DOPA. During stage 2 (2-7 days), mature types of afferent synapses, taste pores, type I cells and type II cells appeared in the taste buds, and fluorescent cells also appeared following treatment of 5-HTP or L-DOPA. During stage 3 (14-21 days), the gustatory cells underwent ultrastructural changes following injection of 5-HTP; i.e. small dense-cored vesicles (30-60 nm) appeared scattered throughout the cytoplasm and were found to intermingle with small clear vesicles accumulated at the presynaptic membranes of afferent synapses, and the electron densities of large dense-cored vesicles (80-100 nm) were elevated as compared with those of untreated mice. Consequently the ability of gustatory cells to take up amine-precursors started simultaneously with the formation of taste pores and mature afferent synapses between the gustatory cells and the sensory nerves.