Presence and structure of gap junctions have been studied at the ultrastructural level in the central nervous system of the freshwater snail Lymnaea stagnalis. Gap junctions are clearly visible in thin sections of glutaraldehyde-fixed and phosphotungstic acid-stained material as well as in freeze-fracture preparations. Various types can be distinguished on the basis of junctional size and shape, and numerical density, diameter, spacing and arrangement of junctional particles. Junctions are present between neurones and between glial cells. The neuroendocrine ovulation-hormone-producing caudodorsal cells show gap junctions at four sites, viz. between somata, between axons in the "loop area" and in the intercerebral commissure and between neurohaemal axon terminals. These junctions show common characteristics as to numerical density, diameter, spacing and arrangement of junctional particles, but morphometry reveals different mean values for junctional size and numerical density. Values are the highest for axons in the loop area, intermediate for axons crossing the commissure, and the lowest for somata and axon terminals. It is proposed that the gap junctions particularly play a role in electrotonic intercellular coupling. The results strongly suggest that the gap junctions between the caudodorsal cells-especially those in the loop area and between the crossing axons-are the morphological correlates of the previously demonstrated electrotonic coupling between these cells. This coupling may enable all cells of the network to act synchronously so that a large amount of ovulation-hormone can be released within a short period of electrical activity (the discharge).