Monoaminergic nerves are characterized by the presence of a population of small synaptic vesicles (40-60 nm in diameter) containing a few large vesicles (80-90 nm in diameter). Thus, although both types of vesicles contain monoamines, the small vesicles must be considered as the organoid responsible for the storage and release of the neurotransmitter, whereas the large ones possibly are involved in the modulation of the process. The small vesicles are electron-lucent or have an osmiophilic electron-dense core that is always linked to the vesicle membrane. Considering morphological and histochemical evidence under different experimental conditions, we proposed the existence of two compartments in the small vesicles: the core and the matrix, corresponding respectively to the electron-dense core and the electron-lucent space between the core and the vesicle membrane in osmium tetroxide fixations. The sizes of both compartments are inversely related, i.e., the smaller the core, the larger the matrix and vice versa. The core even disappears, giving way to a small electron-lucent vesicle made exclusively by the matrix. Thus, the matrix is a constant component of the vesicle, whereas the core is a transient one. Each compartment has a different pool of amine: a loosely bound, easily releasable pool in the matrix and a tightly bound, more resistant pool in the core. These two pools subserve, respectively, a tonic or phasic release of the neurotransmitter, correlated with a tonic or phasic stimulation of the receptor. The core may be considered as a storage or reserve pool. Experimental evidence from our laboratory supports the concept that different mechanisms are operative in both compartments in the release of the neurotransmitter. For instance, a Ca2(+)-independent release would be primarily concerned with the neurotransmitter contained in the matrix, and a Ca2(+)-dependent efflux would be primarily related with the neurotransmitter stored in the core. However, it still must be established that a simple relationship exists between each kind of stimulus and each vesicle compartment, rather than both compartments being integrated in a dynamic functional unit.