Nucleo-cytoplasmic transfer rates of RNA were used as indices of protein-synthetic activity in mammalian supraoptic and paraventricular neurones. Normal, dehydrated and lactating rats were exposed, for varying periods of time, to tritiated uridine, and autoradiographs obtained from hypothalamic blocks. Grain count and cytometry enabled a cytoplasmic/nuclear ratio of grain densities to be obtained for each neurosecretory nucleus. Comparison of these ratios, as well as the regression slopes for their increase with time of exposure to the isotope, indicated the presence of a differential response between the supraoptic and paraventricular nuclei, with the former responding more to the stimulus of dehydration, and the latter to that of lactation. The observations substantiate the view that a partial division of labour exists between the two nuclei in the elaboration of each of the mammalian neurohypophysial octapeptides, vasopressin and oxytocin.