In the glomerular layer of the rat main olfactory bulb, we previously reported three chemically defined interneuron groups: GABA-like immunoreactive, calretinin-immunoreactive and Calbindin-D28k-immunoreactive groups [Kosaka K. et al. (1995) Neurosci. Res. 23, 73-88]. In the present study, we analysed the structural features of these three neuron groups using confocal laser scanning light microscopy, focusing on their dendritic arborization pattern, especially on their close apposition to olfactory receptor terminals labeled by olfactory marker protein. Each glomerulus consisted of two zones, the olfactory nerve zone and the non-olfactory nerve zone. The former was mainly occupied by olfactory nerve preterminals and terminals as well as their targets, postsynaptic fine dendritic portions of intrinsic neurons. The latter non-olfactory nerve zone was occupied mainly by olfactory marker protein-negative profiles. Processes of GABAergic neurons and those of one of their subpopulations, tyrosine hydroxylase-immunoreactive neurons, were numerous both in the olfactory nerve and non-olfactory nerve zones, resulting in their frequent close apposition to olfactory marker protein-immunoreactive elements. Combined confocal laser scanning light microscopic electron microscopic examination revealed synaptic contacts from olfactory nerve terminals on tyrosine hydroxylase-immunoreactive processes at these sites of close apposition. In contrast, calretinin-immunoreactive and Calbindin-D28k-immunoreactive processes, particularly Calbindin-D28k-immunoreactive ones, were distributed almost exclusively in the non-olfactory nerve zone, as if they avoided the olfactory nerve zone, showing a net or honeycomb pattern. Thus, calretinin-immunoreactive and Calbindin-D28k-immunoreactive processes were not or very rarely closely apposed to olfactory nerve terminals. These findings suggested that there might be some differences among chemically defined interneuronal groups in their synaptic contacts from olfactory nerves. Further quantitative image analysis clearly exhibited the prominent differences among these neuron groups in their intraglomerular dendritic arborization in relation with the olfactory nerve zone, i.e. the percentages of the area in the olfactory nerve zone occupied by GABAergic and tyrosine hydroxylase-immunoreactive processes were about 10%, respectively, whereas those of calretinin-immunoreactive and Calbindin-D28k-immunoreactive processes were only about 1% and 0.3%, respectively. These findings suggested that so-called periglomerular cells in glomeruli might be heterogeneous not only in their chemical nature, but also in their dendritic arborization pattern and synaptic contacts from olfactory nerve terminals.