A novel pathway between the subfornical organ and the supraoptic nucleus involving the collateral branches of cell bodies situated in the lamina terminalis has been studied. Fluorogold was injected into the supraoptic nucleus and rhodamine-labelled microspheres into the subfornical organ of rats. Nineteen % of neurons in the median preoptic nucleus and 30% of neurons in the OVLT projecting to the subfornical organ also had axons extending to the supraoptic nucleus. These pathways may represent a novel trajectory for the rely of information from the lamina terminalis to the supraoptic nucleus.