A series of somatostatin analogues with varying activities have been studied by 1H NMR in CD3OH at low temperature in order to find a possible structural explanation for the differentiation of biological activities. In somatostatin analogues with GH release inhibitory activity a beta-turn/beta-sheet backbone conformation is present, which is shown to be characteristic of somatostatin-derived peptides exhibiting this biological activity. On the other hand, among the analogues with antitumor activity, a deviation from these typical structural features is clearly observed, but not general conformational model can be proposed.