The profiles of the neurosecretory granules were measured in the supraoptic nucleus and in axons, swellings and endings in the neurohypophysis. This was performed after the secretion of vasopressin had been fully suppressed by water load for 1.5, 2.5 and 24 h, after the secretion had been stimulated by water deprival for 72 h and in normal rats with water ad libitum. A finite-difference algorithm was used for unfolding the size distribution of neurosecretory granules from the observed profiles, and showed a clear grouping of the neurosecretory granules into 3 size groups: small, medium-sized and large granules. Estimates of the relative volumes of the granules from these 3 groups were combined with previous results. In the supraoptic nucleus, water deprival increased the total volumes of the small and medium-sized granules, and water load decreased the volumes of the medium-sized and especially the large granules. However, in the neurohypophysis, water deprival decreased and water load increased the volume densities of the small and medium-sized, but did not influence that of the large granules. The results indicate that the medium-sized granules are vasopressinergic and the large oxytocinergic. Moreover, the results are consistent with the hypothesis that the small contain another peptide.