In a well-defined sector of adult rat hippocampus (CA1, stratum radiatum), the ultrastructural features of acetylcholine (ACh), noradrenaline (NA), serotonin (5-HT) and GABA axon terminals (varicosities) were compared by electron microscopy after immunostaining with antibodies against choline acetyltransferase, NA, 5-HT and glutamic acid decarboxylase. Approximately 100 sectional profiles of each type were analyzed for size, presence of a synaptic membrane specialization (synaptic incidence) and composition of the microenvironment. An equivalent number of immunonegative varicosity profiles selected at random from the same micrographs were similarly examined. ACh, NA and 5-HT varicosity profiles were of comparable size, and significantly smaller than GABA profiles. They exhibited a low frequency of junctional specialization, amounting to 7%, 15% and 21%, respectively, when extrapolated to the whole volume of these terminals. In contrast, GABA varicosities appeared entirely synaptic. The ACh, NA and 5-HT varicosities also differed from their GABA counterparts in being juxtaposed to a greater number of unlabeled axonal varicosities and a lower number of dendritic branches. In addition, the microenvironment of immunostained terminals showed a much lower number of dendritic spines than that of immunonegative varicosities. This latter finding was viewed as another indication that predominantly asynaptic varicosities do not maintain particular relationships with their immediate surround. It was also concluded that volume transmission represents a major mode of transmission for ACh, NA and 5-HT in adult rat hippocampus, thus contributing to the properties and functions assigned to these transmitters in this part of brain.