The activation of the peptidergic neurosecretory system in Diphyllobothrium dendriticum was studied following cultivation of plerocercoids for short times in vitro and in vivo. In the plerocercoid the neurosecretory cells gave a very weak reaction with paraldehyde fuchsin (PAF). After cultivation for 1 h large numbers of neurosecretory cells filled with PAF-positive granules were evident. The significance of the activation of the neurosecretory system during the transfer of the worm from the cold-blooded fish host to the warm-blooded final host is discussed.