The reduced amount of neurosecretory material (NSM) that is stored in pituitary glands of hereditary hypothalamic diabetes insipidus (DI) rats compared with glands of normal rats reflects their oxytocin content, since DI rats are apparently unable to synthesize vasopressin. If oxytocin were synthesized exclusively in the paraventricular nucleus (PVN), then complete destruction of this nucleus in DI rats should result in the disappearance of all NSM from the pars nervosa. Complete bilateral ablation of the PVN in DI rats was achieved with lesioning electrodes in only 2 out of 24 animals, with varying degrees of destruction of the PVN in the remainder. The rats were sacrificed one month after lesioning and the amount of NSM remaining in the pars nervosa was assessed histologically. In general, the results show a rough, direct correlation between the number of PVN cells remaining in the hypothalamus and the amount of stored NSM in the pituitary. Nevertheless, the 2 rats with complete PVN ablation still had pituitary NSM. These results suggest that although the PVN may be primarily concerned with oxytocin synthesis, it does not seem to be the exclusive site of oxytocin production. Considerable amounts of this hormone are apparently synthesized elsewhere, the supraoptic nucleus being the most likely source.