The total number and the numerical ratio of extrinsic and intrinsic (local) innervations of magnocellular neurons in the paraventricular nucleus (mPVN) were determined after surgical isolation of the nucleus in rats. Various lesions and transections of fibers running to the mPVN were performed to determine the number and possible sources of septal, hippocampal and caudal periventricular fibers to the mPVN. The relatively high proportion of possibly intrinsic connections (43%) suggests a local, integrative function of neuronal activity in the PVN. On the average, 57% of the total number of presynaptic boutons have been found to originate from outside the nucleus (extrinsic afferentation). Only 7% of these fibers ascend from caudal through the periventricular area. mPVN afferents originating from the ventral subiculum and from the lateral septal nucleus comprise about 3 and 5% of the extrinsic afferentation, respectively.