The evolution of vesicular zinc-containing boutons in the developing rabbit hippocampus has been studied during early postnatal life using the selenite-Danscher histochemical method. By P3, large immature mossy fiber boutons with labeled synaptic vesicles were seen in the hilus of the dentate gyrus and in the stratum lucidum of the CA3-CA4 hippocampal areas. After P5, smaller boutons with labeled vesicles were identified in the stratum oriens and stratum radiatum of all hippocampal areas, and even transiently in the stratum lacunosum of P6 animals. Vesicular zinc-containing boutons increased in number and underwent ultrastructural maturation; light microscope densitometric-volumetric measurements were used to quantify their presence in every hippocampal lamina. Electron microscope stereologic analysis permitted accurate estimation of the actual numbers along early postnatal development. Three main phases of zinc-positive bouton growth were detected during the first postnatal month. The first phase, starting at P5, is characterised by an abrupt rise in vesicular zinc content which at P8-P9 begins to decrease. The second phase is characterised by a consistent rise in vesicular zinc content from P10 to P12 to a level which is maintained until P18; this steady period is the result of partial and sequential elimination of zinc-positive boutons in some areas (i.e. oriens of CA3 by P11, radiatum of CA3 by P13, and radiatum of CA1 and lucidum of CA3 by P15) while they continue to increase in other areas. The final phase is a continuous increase to almost adult levels.